Dear sonjanoel: There are several POSSIBLE explanations for your abnormal bleeding pattern. Once pregnancy is ruled out, the most common reason for a menstrual pattern like you have described is not having an ovulation in the last month or so.
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—as either a missed period or prolonged, erratic bleeding as the lining is shed irregularly.
There can be many triggers for not ovulating: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with family or money), increased body weight, anorexia, rotating shifts at work, etc.
Some other causes can be a pelvic infection (doubt this if you are monogamous) or a polyp of the lining of the uterus (possible given that you had erratic bleeding problems on the hormonal birth control as well).
PLEASE get back in to see the GYN or clinic that inserted your Paragard IUD. They can do a pregnancy test and check for an ovarian cyst. They MIGHT try empirical treatment with antibiotics if an inflamed uterine lining is suspected. Occasionally even ibuprofen has been tried for IUD caused breakthrough bleeding. Checking for a problem with a polyp requires more invasive testing so you may be treated with medications initially.
Dang, I hope this resolves as abruptly as it appeared.
Yours,
Jane