6 weeks late

I'm a little over 6 weeks late for my period -and, this week especially I have been EXTREMELY nauseous (can barely eat) and have sharp pains (or could be cramps) that come and go on my left side around my ovary. Sometimes the pain goes across my entire stomach too. My lower back is aching all the time. My breasts are also a little sensitive. I've also had diarrhea like crazy for the past three days. All the hpt I've taken have come up negative though. Could I be pregnant and it's just not showing up on a hpt or should I just give up? Has this happened to any of you ladies?