even with twins your uterus should go back to normal size in just days. Your abdominal muscles will be stretched to heck though and those will take some time to get back to normal.
If by chance it is your uterus, this is serious and you need to be seen soon.
After 7 pregnancies, my stomach is all stretched out and the even though it is squishy(from a layer of fat), the muscles underneath are firm just stretched out.
Also, after surgery, most people have a LOT of extra fluids and will be swollen and bloated for a few weeks afterwards. (blah!)
The hardness you are feeling in and around your scar sounds normal though. Just healing and stuff. Watch for redness, hot to the touch, unexplained swelling, stuff like that.....that would indicate an infection.