Originally Posted by roadbuzz

Without watching you do them, this is a guess, but there's a pair of muscles that run from the lumbar region of your spine to the top front of your thighs called the psoas (actually illiopsoas, pronounced so-ahs). You're probably using them, rather than your abs when you do sit-ups, and putting a lot of strain on your lower back. (It's not necessarily a back strength issue... you're sitting up by pulling on the lower part of your spine rather than the front of your rib cage.) Do crunches for the abs, and include core conditioning on fitball type stuff.