IUD Confusion

I'm really struggling here... so my boyfriend and I had sex over a month ago without a condom, but I have an IUD. I've had it since March 14th, 2016. Right after that time we had sex, around November 12th, I started my period the next day and had it for a week. Now, it's a month since then (I'm on day 35 of my cycle), and my period still hasn't started. Should I be worried that I'm now a week late? I took two pregnancy tests just in case, and they both were negative, but I'm still stressing myself out about it. Am I overreacting? 😂