Pregnant?

Okay. so I had sex with my boyfriend and he never pulled out, no condom or anything. But then a few days later I was supposed to start my period and was just very light spotting for like 3 days but not constant. I've taken about 20 pregnancy tests and all of them have been negative. So Idk what to do. I'm suppose to get my period again in 2 weeks from now, I didn't wanna wait until then because it'll be over a month. 
I've felt nauseas and my lower back hurts every now and then. Is it possible that I'm pregnant but just my hcg levels aren't high enough to read?