My guess is ligament and fascial stretching, especially since you had a c-section there is probably a fair amount of soft tissue binding / scarring that is being stretched right now. I would get some unscented massage oil (or massage oil especially made for pregnant mamas) and do some gentle, yummy belly massage or get a pregnancy massage and ask them to do some gentle work on your belly. During my current pregnancy with twins, I have been massaging my belly a lot :)
Based on your statement about your OB, personally, I would consider finding one who understands me better and could address these kinds of issues with more compassion. Just a thought...