I've had a similar experience, but I'm a bit further along. Mine was put in during Oct 08 and I was still nursing. I had minor breakthrough bleeding (sounds like what you are experiencing) and I had already read in teh Mirena brochure that is normal. That lasted about 2 months and then calmed down and I had very little. BUT about a month ago, I started bleeding much more heavily, like the first day of a regular period, and it lasted a week. I thought, "Oh, I guess I now just had a regular period instead of all that little bleeding." OK. And THEN 10 days later, it started AGAIN - heavy just like the last time. It has now bee over 1.5 weeks and it is still going strong. I actually just talked to my OB about it last night, as I see her socially outside of the office. She said that this is the most common reason she takes the Mirenas out. (Not what i wanted to hear.) It is very common. Makes me feel better that nothing is wrong, but I really loved the idea of 5 year birth control and of keeping the hormones in my uterus rather than in my bloodstream, so I am really hoping that it pans out and calms down. she said that for most women, it does calm down after awhile and the bleeding becomes a lot less and a bit more regular, but for some women it doesn't - it just keeps on and on and they end up taking it out. I haven't had "relations" with my hubby in almost a month. Not good!
As far as pregnancy, I wouldn't worry. She also happened to mention that she just had for the very first time one of her patients (who had the Mirena put in) become pregnant. She has been practicing for over 10 years and has TONS of patients and this is the first time. It looks like this woman actually lost hers at some point. They have done both an ultrasound and an MRI (to protect the baby, they need to figure out where that bugger is!) and they cannot find the Mirena anywhere in her body. Their theory is that it expelled at some point and somehow the mother did not notice.
So, no worries. She told me just to check that the string is there after you have a heavy period and you should be fine. Hopefully we will both stop bleeding soon!! :) No worries.
By the way, I saw another person's response about using the natural method. I'm glad it works for her (and I appreciate what she says about chemicals, I agree with her on many levels), but I have to tell you that my OB friend also said "Yes, lots of my patients used that method!" In other words, they became her patients because they got pregnant. :) I wouldn't mind a little "surprise," but it sounds like you are pretty anxious about not getting pregnant. To that end, the Mirena has a higher percentage of keeping us NOT pregnant than any other form of birth control. And it works really well for most women. Good luck!