No period, cramping, backpain, neg. test helpp ):?

I haven't gotten my period since the first week of March. The last week of April I was bleeding but it was very light, brownish, and only lasted about 2 days on & off. Lately, the past two weeks I feel as if I'm going to get my period (bad cramping, hurt, etc) but I've also been feeling nausious sometimes, & I've been having back discomfort (nothing really painful.) I took three tests, all about 2 weeks apart, all came back negative. I don't understand how I could get pregnant. Me and my boyfriend always used a condom & also pulled out w a condom. Help ))):