Why am I nauseous?

I recently thought I was pregnant, was 10 days late and on the 10th day I finally got AF but it really only lasted 2 days so I took a pregnancy test after I finished my period and it was negative so I don't believe I'm pregnant but here I am 4 days after finishing my period and I've been so nauseous, extremely fatigued and so thirsty. With a few other pregnancy symptoms but I'm pretty sure that if I truly was pregnant it would've shown up on the test I took a few days ago? Anyone else been through something like this and was pregnant after all or should I assume I have a health issue going on?