Could I be pregnant?

I have had an IUD for about 6 months now and it's been working wonderfully for me. My boyfriend and I had sex about a month ago (LDR) and I had my period the next day. Now I'm noticing all sorts of mood swings, fatigue, weight gain and I just feel strange in general. 
Would it be possible to get pregnant w an IUD even if you started the next day after sex?