What Are The Side Effects Of Mirena IUD?

I've had a merina coil fitted for nearly 3 years and only just recently I've started to bleed a little after intercourse not all the time but a few times I've been to see my doctor today I'm due a smear tomorrow and that's going ahead but I have to take a water sample and he's goin to take some swabs and do a test for chlamydia he also said I could have an erosion or bacteria vaginosis can you explain this please thank you