What Causes Tiredness, Nausea And Lower Back Pain While HPT Is Negative?

Hi. I'm a 31 year old female with a 5 month old little girl. I'm really healthy with the exception of high blood pressure. I have a question for you about pregnancy. I think I missed last month period and am due to have one this month this week, but it has not started. I started taking the pills the beginning of January. I am on the pill but a different kind (Nora Be) due to my high blood pressure. I feel pregnant (really really tired, nausea, lower back pain, food cravings, cramps, etc) but I have taken 3 pregnancy tests that have all been negative. Could I still be pregnant?