Could I be pregnant even though three tests said not pregnant?

I have taken three pregnancy tests, two hpt and one at the doctor, but it wasnt a blood test, The first hpt was positive and the rest, including the one at the doctor, were negative. I took them weeks after the last time i had sex, so me taking the tests too early isnt an issue. Now my periods are normally really irregular, My last period was February 10th, and this ]period started March 26th, but this period is very very light. Im nauseous and this has never been an issue or side effect from my period. Im not cramping and my boobs werent sore, which usually occur a few days before my period. Could I be pregnant even though the tests all said not pregnant?