Hmmm... I'd probably need to see it to know for sure, but there's plenty of possibilities.
Have you checked to make sure your gauge/tension is even throughout the project? Having rows that are tighter or looser than others can really throw the shape of a project off.
Are missing or adding stitches as you go? I always used to have an issue keeping the number of dc's the same throughout. Try counting your number of stitches to see if you have the same as you started with. If you're off, you'll have to decide it you can correct the number of stitches or will have to rip back.
Perhaps your initial chain and first row or so are tighter or looser. I crocheted myself a blanket out of sc, but the beginning of it was a lot looser than the end of it, so the blanket never lay quite flat. There's not much you can do about this mid project. If you determine this is the problem, next time you can adjust for this in the beginning.
The good thing about large objects such as blankets or afghans, is that they are so large and have enough weight that they tend to stretch themselves into shape when draped over something or used. As long as it isn't horrible (you know, unusable ect) most people won't notice a little warp here and there. And if it really bothers you, there you can always frog it and start over. Though I know that's no fun.