I have had Mirena for 4 years and honestly have had no bad experience so far. I bled for about a week and a half after I first had it inserted and nothing since then I mean not even a drop of breakthrough bleeding since then but for about two days now I have started to have a small amount of breakthrough  bleeding. Honestly I can really only see it when I wipe and maybe one spot in my underwear. I know that breakthrough bleeding can be common with Mirena but considering that I have went four years and haven't seen anything I want to know if I should be worried or just give it a day or two and see what happens. I have no other symptoms or issues and like I said honestly for me the Mirena has been wonderful but I just want to make sure that I shouldn't worry over these last two days of light spotting. I would like to add how have been at the beach for the last two days so I've been doing a lot of swimming in Beach water and pool water I don't know if maybe this could have brought it about I have went swimming in the past and never had an issue but just really not sure just wanted to see if anybody else has had a similar experience with not having any breakthrough bleeding and then all of a sudden boom it starts.