For uncircumcised men, the foreskin of the penis sometimes becomes tight, usually because of swelling from an infection. Not only is this painful, it can interfere with sex because the head of the penis is not uncovered as it usually is.
You should see a health care provider to determine the exact cause of the problem. If the infection can be easily cleared up, no problem. If, however, there is some other structural problem at work, you may need to have the foreskin removed. This is a fairly simple procedure and recovery is typically rapid, though there may be some pain at first. This is, after all, one of the most sensitive parts of the body!