so, i'm somewhat concerned. usually, my periods last around 4-5 days, with a moderate flow. i just became sexually active this past june, and since then, my periods have not been quite right. my past six periods have all lasted around 2-3 days, will less flow than usual. the color is normal red, and there is sometimes some clotting, but it's not the same as it has been. i'm somewhat irregular, but never have had a shortened period prior to becoming sexually active.
i have PMS symptoms before hand (sore breasts, irritability, headaches, back aches) that go away with the bleeding. also, i have my usual cramps. none of that is out of the ordinary, just the length/flow. i've taken 4 pregnancy tests, all of which were negative. my doctor performed a pelvic exam and also said there were no signs of pregnancy.
i know this might be stress, but this has been going on for around 4 months now. i'm NOT on any form of birth control pills. what can be causing this? please help!