It's just a conceit really - you can either go with it, or add it to the "that's stupid" list.
She'd just had major abdominal surgery which would presumably disable anyone for some time in reality, but the plot required that she be able to run about straight afterwards. So you get a bit of her running about as though nothing was wrong, and then a moment of her crouching over in pain to acknowledge the fact that she'd experienced a major trauma.
What's funny is that I remember consciously saying to myself "kinda silly that she's running around after such a major operation, but I'll just let that go since this movie is already pretty preposterous." So when she'd double over, I just assumed it meant there was another alien-baby.
The only reason I thought a plot screw-up had occurred is because I was allowing another plot screw-up.
I'll admit some fault in interpretation here, but honestly, a post-op patient doesn't suffer sudden, fleeting pain attacks while running about. I'm with steveuk; I would have much rather they explained it away with the "it's the future" catchall, than the comical bouts of abdominal cramping we got.