Cursor will not go to next line will creating text.

I've got this problem in AE (CS 5.5), when writing text, and I come to the end of a line, and I hit return, the cursor goes back to the beginning of the original sentence instead of skipping down to next line below it.
I can write a new sentence but it is on top of the first sentence instead of below it.
Have I clicked some sort of strange cursor parameter?
How do I get it write normally?