Dear An: Given your sexual lifestyle, I would believe the pregnancy test results. Still you did the right thing in testing.
The most common reason for a menstrual pattern like you have described is not having an ovulation every month. In a normal cycle, estrogen is produced all month. Estrogen is responsible for building up the lining of your uterus so you have something to shed each month. The good news is that you have enough estrogen to make a lining that does shed.
In a normal cycle, progesterone production increases following ovulation and release of an egg.. Progesterone "stabilizes" the uterine lining in preparation for a possible implantation of a new pregnancy. If you are not pregnant that month the levels of estrogen and progesterone fall, triggering the release of the uterine lining—your period.
So, if you have missed an ovulation, the estrogen build up of the lining continues, but without the usual ovulation associated progesterone. Thus, the hormone levels don't decline, and the lining stays up inside the uterus— as a missed/late period. Alternatively, parts of the built up lining can begin to shed creating erratic and/or too-light flow.
Causes for not ovulating are multifold: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with boyfriends/girlfriends, finances), increased body weight, anorexia, rotating shifts at work, etc.
Given that your flow change was one time only, the most likely candidates would be an ovarian cyst (can cause pelvic pain also), or some kind of acute stress. As you read above, low thyroid can interfere with regular ovulations; it can also be linked to chronic constipation.
As for the constipation, it in itself, can create abdominal bloating. It is not easy to know the cause of the constipation--can be diet, dehydration from summer heat, decreased exercise, certain medications, etc.
If the bloating continues despite the laxative induced bowel movement you should be seen by your MD or clinic. If your menstrual cycle does not reset itself naturally over the next month (and especially if there is pain of an ovarian cyst) you should see a GYN for the most "for sure" diagnosis.
Yours,
Jane