Why do sit ups hurt?

The other day, I had Miss E on the floor beside me.
She was entertaining herself with various toys, and I'm trying to get back on track with the whole exercise thing, so I grabbed my weights and got to work.
After my set amount of reps with the weights, I decided to do a few push ups and sit ups.
When I was a young buck, sit ups were an everyday occurrence while serving my Country, so why not start slow again?

After about thirty sit ups, I began to feel a slight twitch in my abdomen. I kept going, focusing on my technique, as to not mess up my back further, managing to do a few more.
All was well, until I tried to sit up. I swear to everything holy, I actually felt like I had a hernia.
I stood up as straight as I could, allowing my abdominal muscles to stretch, but I had these two burning, nagging spots in my belly. What made things worse, was the fact that I had already picked up Ellery, and was frozen in severe pain. She knew something was wrong. She looked at me, trying to figure out why daddy looked like he was getting shanked in a prison shower, but no words would come out. I just stood there, bend slightly to the left, making my peace with God.
Then, as fast as the pain hit me, it disappeared into thin air.

Lesson learned. Abs are over-rated.