sit-ups, leg lifts, other ab exercises cause weird pain?

[Nell-e-nel]

I like to exercise, but every time I do ab exercises such as sit-ups, leg lifts, or most ab exercises; I get this weird abdominal pain right between my belly button and my hip bone. It feels almost like a cramp. I was kind of scared I might be an internal organ or something. I've had this pain when I work out my abs for at least 4 years. It usually doesn't bother me after I finish my work out, but recently the pain has lingered a day or two after my workout. Please help!
Oh, and the pain only occurs on my left side.