Period 35 days late?

Haven't had a period since November 25th but all pregnancy tests have come back negative.. my period is very regular (almost down to the hour) but I'm so unsure of what's going on. No stress, diet or exercise changes, medications or anything that would make an excuse.. is this something to call my doctor over? Has this ever happened to anyone and they ended up being pregnant? If I were to be pregnant I'd already be 10 weeks!