Mirena and condom

I'm terrified and anxious....

I've been on the mirena iud for a little over a year now and I haven't had sex till recently. My last period was in the beginning of November and I had intercourse on December 11th (we used a condom). And yesterday I had a period but it only lasted for a day. What does this mean??? I'm in college and I was completely safe however I have it on my mind that I could possibly be pg.