I have pain when I sit up from laying down or when I twist wrong. I'm pretty sure it's the ligaments stretching from my growing belly.
I get aches in my stomach....usually on one side or the other. Mine are ligament pains. If I move a certain way it gets really sore and stays that way for awhile. Where is your pain in relation to where you do the heparin injections? It could be they are hitting a series of nerves or it could just be ligament pains.
If I get up or twist, I get a pain...but I also get the skin stretching/belly button pain. My skin is painful around my belly button and sometimes on the sides. It's painful to the touch sometimes & my dr said that it's normal, especially if you were pretty thin before.