I went to the gym the other day and did allot of sit-ups my stomach is very tight and hurts?

DOMS. You have delayed onset muscle soreness (doms). It is one of the most common forms of sports injury (cheung, 2003). The pain typically worsens over 24-72 hours and usually resolves within 7 days. Nsaids may be effective for the soreness. You should be able to return to sit-ups after 7 days but you should start gradually. You can exercise other muscle groups while waiting for doms to resolve.
Have muscle strain. The purpose of sit-ups is to make abdominal muscle stroger with time. The rule to do exercise is to them in a very graded fashion depending on your age. Now do nothing for next three weeks and use cold packs for ten minutes two time a day for a week. Your problem will go away and muscles will recover.