A guppy of mine swam oddly before when she was heavily gravid, although it was her tail end that drifted upwards slightly when she slowed down, rather than her head. It turned out to be because the fry were pressing on her swim bladder, and she returned to normal right after having her fry.
I'm not saying that that is what's happening with yours, it's not something you can easily tell, and it certainly doesn't happen every time a livebearer is gravid. I'm just saying to try not to panic, because there's a possibility that that can happen.
Have you done a water test? Done a water change recently? How large is the tank, and how many of which species do you have?
