Tell me this: Did you go from zero situps to doing 50 situps?
If yes, my gosh, you don't have to do it that way.
Tell ya what, try working up slowly. 20 situps is plenty, maybe even fewer! Don't do them every day at first. Every other day is fine, but especially if your muscles are still in bad pain, don't do more situps.
It's natural to feel sore if you've been working muscles more than they are used to, but a lot of folks overdo things when they start and then give up because it's too hard.
No need to hurt yourself that badly. "No pain no gain" doesn't therefore mean "more pain, more gain."