I would recommend that you see your OB/GYN or whoever placed the mirena device. While most people have no trouble with the device, there can be some complications. Your issue sounds as if it is troublesome -- therefore require evaluation.
A Mirena device is an intrauterine device which is loaded with progesterone -- a female hormone that is slowly relased over the 10 year lifespan of the device and therefore works to inhibit a pregnancy. Most people will subsequently have a lighthening of there periods and potentially even stopping of having periods. Irregular bleeding
can also happen, but this should have resolved by a year.
There are some rare but real complications to consider. Firstly the device can be a source of infection. Pelvic inflammatory disease is a real and serious complication that should be ruled out. The device can also move -- which can therefore cause problems. Rarely, it can puncture through the uterus -- again a major problem. There are also rare cases of an ectopic pregnancy -- a pregnancy where the fetus is located not in the uterus but in another spot -- again a problem.
Your situation is worrisome. Talk to your OB/GYN -- he or she can help evaluate and potentially treat this. Good luck!