Can I Still Be Pregnant But With Negative Test Results?

My boyfriend and I had unprotected sex maybe 3 times this month. He always pulls out, we never do it twice in one night and I make sure he hasn t ejaculated that day (to be sure there are no live sperm left over in the urethra). My period is usually like clock work, but this month I have been extremely stressed. My period hasn t showed in 10 days. I ve taken 3 pregnancy tests and all came out negative. I have sore breasts (which is normal for me) and what seems like pms cramps. But still no period. Am I pregnant? Am I getting false negatives because hcg is too low at this time? HELP!