For three months now, a few days after my period ends I get unbearable cramps, exactly like menstrual cramps, which last the whole month...any ideas?

Why not give info? Your age, your pregnancy history, and what you're doing for contraception are among the top 3 things i need to know to help you figure out what's going on.
Post menses cramps. It's usually considered atypical for a women to experience menstrual cramps after her period has ended,but some of the more common reasons why this happens are irregular or painful Ovulation,recent insertion of certain IUD birth control devises and endometriosis.Ovarian Cysts and fibroids may also be to blame. Also infections. Persistent pain past the end of your cycle should prompt you to see DR.