Pregnancy with Mirena?

I had mirena placed two and a half months ago. Had unprotected sex before placement and my doctor said I had a negative pregnancy test. They also told me to wait two weeks before sex. Other women with mirena said their doctors told them 6 weeks to two months. I've also heard to use protection for the first year. The last few days ive been having pregnancy symptoms. Nausea, tiredness, mood swings, no period, granted I do have a four month old. I was also reading on baby center about women who got pregnant and their was quite a few stories. So my question is has this happened to anyone else and did it turn out to be pregnancy or something else?