I am 12 days late today...?

and I keep getting negative HPT's. What is going on? My husband and I have been TTC for about 3 months now, my last period was Oct.16th. I am usually like clockwork. I am so confused on what is going on. Has this happened to anyone and you were actually pregnant? With our 1st son it showed up right away! I've been feeling a little sick, no tenderness tho. a little crampy with a lot of lower back pain. Any advice would be great!! Thanks!