Positive then negative?

Ka
So on Saturday (11DPO) I got a faint positive with a FRER, and then today, Sunday (12DPO) I took another FRER and it was negative.. I thought I might have saw a line but it definitely wasn't as much as the first test and I can't say for sure it's even there..
I was planning on going to the doctor tomorrow for blood test just to find out one way or the other because I really can't handle the stress right now of waiting and wondering :(
What do you think? Could I actually be pregnant? I've had a chemical before and this just does not have the same signs and symptoms so I don't think it's that. I'm either pregnant or I never was. But it's seriously consuming my thoughts and I want to just relax but I have anxiety so it's really hard too..
Comments? Experience? Advice?