What Causes Lower Abdominal Pain A Few Days After Period?

i normaly get my periods on th 24th. But, for aobut one year now i have pain after 12-18 days of my periods. It came once in few months earlier, but now for about five months now, it comes every months. and it was lasted only for less than five minutes earlier, but now it lasts for around five to 15 minutes and very painful. The pain comes from just below the belly button to the below . so, what should it be the reason? I am having a fertility problem as well, and the doctors found by a SHG test that, there is a blockage in the right tube as well. pls answer me.