9 dpo

Has anyone had a negative blood test and gone on to actually be pregnant? I've been having alot of cramping and sharp pains finally decided to go to the E.R only for them to tell me that they don't know why this is happening and that my pregnancy tests came back negative. I've been nauseous for almost 4 days and extremely tired. I was also told that I have thick endometrial tissue right now and it looks like I'm on my period but I'm not sooo. I'm confused cause I don't have cramping before my period and it's not like this type of shooting pain cramping. Anyone has any info that would be great.