Cramping after intercourse occurs because of contraction of the uterine muscles.  This is normal when there is excitation and orgasm.  But significant cramping after intercourse could be due to endometriosis.  You may use a heating pad afterwards over your lower abdomen or take a hot bath since these aid in muscle relaxation.  Using NSAIDs like ibuprophen can also be helpful for many women.  Decreasing your caffeine intake to only one caffeinated drink per day and staying away from alcohol also helps decrease this cramping for many women. 
Endometriosis also results in very painful cramping during menstruation and causes pain during deep penetration during sex.  If you have any of these symptoms, you should be evaluated for endometriosis at your next gynecological visit.  This is important to do because endometriosis is a major cause of infertility in women and may also lead to intestinal problems if it is severe.  Many women don't realize they have endometriosis until it is already causing problems for them.