I'm on a mirena (levonorgestrel) i bn having back pain heavy cramping and spotting old blood jus a little first day for the blood. i haven't had a period since mid nov?

Breakthrough bleed: Breakthrough bleeding (bleeding at the wrong time) is very common on birth control. It happens in about 1/3 of iud users. Its no big deal and usually self corrects over time. If you are not satisfied or it continues to happen then consider using a different birth control method. It is a common side effect and does not mean something is wrong or that the iud is not working.