What Causes Bloating And Cramping Pains During Menstrual Cycle?

hello! i hope you can help as this has been a problem that has been ongoing for years and I am back and for my gp and gyne consultant who cant seem to give me any answers. i have had 2 sets of blood tests which they said weren't as they'd hoped but to book a hsg and they said i had to ring and book on the first day of my period which i will do but i really want someone to explain what it could be?? i am 22 yrs old 5ft 6inch, 8 stone and have been trying to concieve with my partner of 5 years for the past 2. i also experience regular cramping pains, feel bloated and today i have started my bleed later than expected but its brown?? please help