Has anyone bled what they thought was a period while pregnant?

Yesterday I found out I'm 12 weeks pregnant!!! I bled for a few days when my normal periods were due but was light. I was really stressed so thought that was why they were short and light but on the third month I was getting extremely sick and no "period". Am I the only one who this has happened too? And they baby is perfectly fine, thank god! 
I was on Mirena but somehow it "fell out" and I didn't know...