Oh my.
You should definitely schedule yourself for an ultrasound no matter what it is.
I don't know why you were told that you couldn't have any more children, but sometimes doctors make mistakes and you could be pregnant and would need to find out so that you could start to take proper care of yourself and your growing baby.
If it's something else, then whatever it is that is moving around is certainly not healthy and will drain you of all nutrients and make you very sick.
I would bet more on a pregnancy since it feels to you more like a pregnancy and I do not think that a parasite would have as much of an ability to move like a baby, but of course, only you know how you would feel. I think that parasites would have a marked bad impression on your health as well....such as a tape worm, which would make you hungry all the time and you would start to feel very drained and tired.
Since you are a plus size lady, it is very likely that you wouldn't notice your belly grow either.
Good luck with the ultrasound and keep me updated!