Hi I am 2 months late taken 4 pregnancy tests all negative now getting sore nipples, starting to get really worried that I'm pregnant. Spoke to doctor, they said that my BMI is bit low (5ft 4, 50kg) so might be irregular.
Am I pregnant?

Were you regulary before your period stopped? 50kg (110lbs) is small, but not necessarily unusually so for your height.

The tests you took, were they at the doctor's office or home pregnancy tests?

You most likely should get a blood test done at your doctor's office to cinfirm that you are indeed not pregnant.