I've heard of this glitch a lot before. I even found a (hilariously creepy) video documenting it on YouTube. It's just related to the pregnancy/holding an infant. I wouldn't really worry about it. But, as mentioned before, the only known way to fix it is resetsim.
*Googles it*
Girl Sim stands up in the middle of her bed and her arms are posed as if she is holding a baby, and then she snaps back to bed for a few seconds, and repeats.
From a list of TS3 bugs & glitches. But no solution is given.
So, report back if you manage to fix it.