First of all,do a USG lower abdomen and some hormone analysis with urine test for R/E,M/E to rule out underlying pathology. Then consult with your doctor.
Your last period was 3 wk ago and it is impossible to get visible stretch mark with enlarged abdomen for pregnancy
before 12 wk. So there is no chance of pregnancy in your case.
You may be featured with Pre Menstrual Syndrome (PMS) and do these things like avoid salty food/ coffee in 2nd half of period,Avoid stress
by yoga/deep breathing,Control body wt,Take healthy diet
,Use Tab FLUOXETINE
20mg OD, may need OCP/Progesterone
pill,maintain personal hygiene,drink lots of fluid and need proper sleep.