Hi, I'm 16 Years Old And I Have Had 3

Hi, I'm 16 years old and I have had 3 negative pregnancy tests (all taken after my first period ) and I have had one light period and one normal. Both o my periods were at the right time. But now my stomach seems to be getting bigger and my lower abdomen hurts when I push on it. I have been getting more frequent headaches and heartburn. And I'm trying to figure out if I'm pregnant or if I need to see a doctor. But I don't want to tell my parents unless I have to.