Late and not pregnant?

I'm 31 years old, I've had three successful pregnancies in the past. I'm trying to get pregnant again and I've missed my period but I'm not pregnant! a little background, I was diagnosed with endometriosis at 18, found out I was pregnant for the first time at 19 and had a perfectly healthy pregnancy and gave birth to a beautiful baby boy. Since then I've had two girls. In 2014 I was diagnosed with cervical cancer that has since gone into remission. My husband and I are trying to have another child but it's just not working. I had an early miscarriage in Oct of 2017 (early being within the first 6 weeks) I have completely missed my period but all of the tests that I've taken report negative. I don't know what's going on and I don't know what to do. Any recommendations? Or thoughts as to what is going on?