Our English Bibles are not always referring to Christ just because they use the phrase "angel of the Lord." You need to use context to determine whether a Hebrew malak or Greek angel (literally "messenger" in both cases) is (1) an ordinary human messenger, (2) a created heavenly being, or (3) the divine angel/messenger who is the Lord himself.
In most English Bibles, the translators do the first part of that work for you. If context suggests a human messenger, they translate the word into "messenger" or something similar in English, and most readers never even realize that it's the same word as "angel." But if context suggests either a heavenly being or the Lord himself, our English Bibles usually translate it as "angel" in both cases. So, you need to decide which it is from the context. Is it a created heavenly being, or is it the Lord himself?
In the case of Matthew 28 and Mark 16, the angel announcing Jesus' resurrection does not speak as if he is Jesus himself, so you are right to conclude this was a created heavenly being. In fact, all appearances of "angels" in the gospels and Acts likely are created heavenly beings.
