I don't know what to do!

I've always been SUPER regular. Like clockwork. I had my IUD removed a couple months ago after have it for 2 years and I've continued to be regular. 
I'm late this month, I'm on CD 31 and my cycles have always been 28 days. All my pregnancy tests are negative though. 
What do I do? How long do I wait before going to the doctor? What could be a reason for my period not coming? 
I'm not under any stress or anything unusual!