Pregnant??Or Just freaking out??

So I have an IUD and have not been able to check my strings recently but I'm pretty sure it's still in there. I had unprotected sex with my boyfriend and before we did I was bleeding lightly (I had had my IUD for about 11 days) and after we had sex I only had brown discharge for like four days and now I have nothing?? Is this normal for a new IUD or should I worry that I'm pregnant?