help!!

precious
So I got my AF BUT THE WHOLE TIME I HAVE BEEN MISERABLE IVE BEEN EXTREMELY NASIOUS AND WANTING TO VOMIT 
WHAT IS GOING ON ???? 😩 
A friend told me I could still be pregnant but I'm not convinced She said she was 3 months having periods when she found out 
Has this happened to anyone before???? 
I'm scared