Two positive pregnancy tests. 4 days later, it's negative

Krista
I had a miscarriage in September of this year. I had the feeling that I was pregnant again and that was confirmed 4 days ago. I took two First Response tests, both came out positive. 
Since my miscarriage, I've been worried about this new pregnancy and took the cheap strip test just because. It was negative. I took two more strip tests and both were negative, so I took another First Response, and it too was negative. 
My period is extremely regular and it is now 5 days late. I'm worried and confused. I have a doctor appointment tomorrow, but wanted to see if this has happened to anyone else and, if so, what was the outcome?