Just relax and rest your abs. Take a pain killer like Brufen and anti inflammatory drugs and it will go off! After couple of days you will be fine.
If not go to a Physician for a check up and till the diagnosis do not worry, Take care!
See a doctor. Could be nearly anything, maybe just muscle soreness and you'll get over it in another day or two, but it could be a little hernia; I know someone who got a hernia doing sit-ups once. Lie down flat and relax when you can, and see a doctor soon if the pain keeps on.