I used to go to a Charismatic church where the gift of tongues was believed to still be applicable to today's church (no one used it in church, but at personal devotion). I've been a reformed believer for a while now, but I don't understand the split between churches in believing that gifts such as prophesy and tongues still exist or not.
I used to "speak in tongues" (only in deep prayer by myself when it came out on its own) a few years ago, but I always stop myself when it starts to happen because it seems that Reformed theology (or at least my church) doesn't agree with it. And I am afraid of doing something that goes against God's Spirit (like blasphemy). I haven't done this in about 2 years and it doesn't come up that often. But I feel like I need to know if this is from God or the devil.
I was a Charismatic for about ten years. I spoke in my "prayer language" in private every day during those years. There is no doubt that you will soon be getting a lot of good theological answers on this subject from the guys on this board and I can give you some good ones myself. But for now I'd like to throw something at you that is more experiential. Though it is true that Sola Scriptura is what we hold to and it is also true that we cannot always trust our experiences and/or feelings to learn truth, it is also true that the Bible teaches us that we have a God-given conscience.
Romans 9:1a
I speak the truth in Christ—I am not lying, my conscience confirms it in the Holy Spirit—
Romans 14:23b
and everything that does not come from faith is sin.
It was long before I became a Reformed believer that I began to struggle deep in my heart (conscience) with the idea that my "prayer language" was not from God. I think that I knew "instinctively"... (Boy, if you knew me and my complete story you would know how badly I hate using words like "feelings" and "instinct") but I think I knew instictively that I was born again (before I became a Charismatic) and that I did not have a "tongue" from Satan. In my humble opinion the only thing satanic about the "prayer language" is that it is false. (LOL As if that weren't enough!)
Your case may be different than mine was but I believe it is the same. The Lord is graciously leading you out of that naive stage (PLT!) of your faith and into the historical faith of the Reformers.
You will NEVER blaspheme the Holy Spirit by questioning some experience that you have had whether it came from God or not. The Pharasees were blaspheming the Holy Spirit in the only passage of Scripture that I know of that mentions such a blasphemy. The Pharsees and Saducees and teachers of the Law knew that Jesus was a "teacher come from God."
John 3:1&2
1Now there was a man of the Pharisees named Nicodemus, a member of the Jewish ruling council. 2He came to Jesus at night and said, "Rabbi, we know you are a teacher who has come from God. For no one could perform the miraculous signs you are doing if God were not with him."
Despite this knowledge they claimed that he was possessed by the devil.
Mark 3:22-33
22And the teachers of the law who came down from Jerusalem said, "He is possessed by Beelzebub[c]! By the prince of demons he is driving out demons."
23So Jesus called them and spoke to them in parables: "How can Satan drive out Satan? 24If a kingdom is divided against itself, that kingdom cannot stand. 25If a house is divided against itself, that house cannot stand. 26And if Satan opposes himself and is divided, he cannot stand; his end has come. 27In fact, no one can enter a strong man's house and carry off his possessions unless he first ties up the strong man. Then he can rob his house. 28I tell you the truth, all the sins and blasphemies of men will be forgiven them. 29But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin."
30He said this because they were saying, "He has an evil spirit."
Questioning whether or not something is from God is not blasphemy. I believe that blasphemy of the Holy Spirit is knowingly calliong the work of the Holy Spirit, the work of Satan. (I'm not even sure that it can still be done since Jesus has left to be with the Father) The more you move into the Reformed faith the more you will question many things that you used to believe. It's fun actually. Relax and enjoy the ride! Take it from an ex-tongue-talking, pew-jumping fanatic.
BTW I may upset some "Reformed Charismatics" by saying this, but when I was a member of a Yahoo group (no longer around

) called "ExCharisma" I was trying to turn to Reformed belief and still keep one foot in Charismaina (Sound harsh? I almost typed Sodom). When I first discovered John Piper I wanted the guys at ExCharisma to tell me why we couldn't be Reformed and Charismatic (Piper - though I respect him - leans that way a triffle). One of the wisest things I ever heard from ExCharisma came from moderator Robin Arnaud. Robin said (I'm paraphrasing), "Kevin, Piper hasn't been where we have or seen what we've seen." And you know what? Robin is probably right.