Dear o0_Rayne_0o: If you have the Mirena type of IUD then the unpredictable bleeding patterns maybe a side effect of the hormones in the IUD. They synthetic progesterone in the Mirena makes the lining of the uterus thinner. In some women this can create a less stable lining which is prone to unpredictable bleeds.
If you have the copper IUD (eg Paragard) then 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 can stay up inside the uterus as a missed/late 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 things which alter ovulations: low thyroid, 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.
Lastly, the midcycle PMS type symptoms may actually be a result of the hormonal shifts which precede ovulation. There is a rapid shift in estrogen levels right before the egg is released.
I would urge you to contact the GYN or clinic that inserted your IUD. They can give you the most "for sure" answer after a thorough history. It may be something as normal as midcycle ovulation symptoms.
Yours,
Jane