I havent had a period in 4 months, pretty sure I'm preggers but my test keep saying negative?

I have went to my PCP and the hospital because I keep getting sick. I am sure I'm pregnant but the pregnancy test all come back negative. What should I do? Its been 4 months and I'm running out of options. I have symptoms like crazy! I'm lost, I never been pregnant before…Im 24 and engaged. All suggestions are appreciated.