PhD students reviewing papers
There are several possibilities. One is that you are lucky. One is that they are desperate (no, not really). But, I think that the most likely answer is that you have been "seen" by an editor via your conference papers and seem to be a good candidate to review a particular work. I would guess that the editor sees a match of interest and topic that points to you. 

If you have the time for it, it would probably be a good idea to accept, just for the experience. 

But, no, I don't think it is especially "normal" in most fields. When your advisor is an editor or a close associate of an editor, it might be more common, of course. Perhaps someone like that suggested you.