My lower back hurts when I do situps?

Sounds like you could be having some issues with both the spine and the joints. Better see a doctor and ask about it. It could be something as simple as the fact that you may be doing the excercise wrong or it could be something serious that you need to be seen about at the doctor's office. Just don't do it again until you've seen a doctor. You sure don't want to do more damage to yourself.