Confused....

Me and my boyfriend have been trying to get pregnant for the last month, and I haven't been on any type of birth control prior to trying to conceive, other than using condoms. I don't have any medical history that could cause me not to get pregnant. So I have missed my period, and have cramp pains almost like I'm going to get my period, but it doesn't come. I am only a week late, and have taken 4 HPT's and all have come up negitive..I have never been late before..my cycle is usually between 28-30 days...So, what I'm wondering is, can I still be pregnant even though the tests are all coming out negitive?? And if not, what could cause me to miss my period even though, I'm not stressed...Thanks!