This may sound weird, but...

TaylorandMel

I woke up last night and the right side of my uterus was hard, like something was pushing out, while the left side was not (I am 18 weeks pregnant with twins, I have not been pregnant before, and I haven't felt movement yet).  I massaged the area lightly, and it went away after a few minutes, both heart beats were good with the doppler.  Has anyone experienced this?  It really freaked me out. TIA.

gtbride&groom

Mine feels like that when one baby is close to the skin.  At times it feels like there's a ball inside.  I won't freak out at all.  It will start happening more often. I joke with my husband that one of my girls is trying to come out through my belly button.

Torani

Just one of the babies.  I often look down and my tummy looks uneven because the baby is turned a certain way and pushing on one side or the other.  You will notice it more and more.  Once you get bigger, you will be able to feel where body parts are sometimes too.  Not weird at all!