Dear blahhh: It is less likely that chlamydia (especially since you got treatment right away) would be the culprit in your "too-light" spotty flow. According to my calculations, if your period was due 8/12 you would have been ovulating around 7/28. Yet if you had conceived, a pregnancy test should have been reliable by 8/19.
If you are truly NOT pregnant, 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 most times.
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 do not ovulate, 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—your missed period. Alternatively the built up lining can begin to shed erratically under its own weight creating erratic spotting.
There can be MANY causes for not ovulating: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with boyfriends/girlfriends, money), increased body weight, anorexia, rotating shifts at work, etc.
Blahh, if you have not had a flow by mid-September, or develop pelvic pain (eg ovarian cyst) see your GYN or local family planning clinic. It would also be a good idea to get a repeat pregnancy test done at a GYN or clinic since you are having pregnancy-type symptoms.
Yours,
Jane