Dear chelsea: If you happen to be on some kind of hormonal birth control (pill/patch/ring/Mirena) the most likely explanation would be "normal" breakthrough bleeding/spotting due to the hormones in the birth control. However, if you are not using hormones then the following is my best GUESS.
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.
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 on its own
creating erratic bleeding patterns which are usually "too-light" or super heavy and prolonged.
There can be many causes for not ovulating: low thyroid, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with relationships, money worries, etc.) increased body weight, anorexia, rotating shifts at work, etc.
Chelsea, after almost five months of this you deserve the most "for sure" answer. A GYN or your local family planning clinic can do an evaluation. If you are about 23 years old (ie were born in 1989) the risk of worrisome GYN conditions such as cancer is very remote.
In Support,
Jane