Faint positive then negative?

I thought I started my period on March 27, bled for only 3 days and it was much lighter tha usual, and ended abruptly which is very abnormal for me. My friend and mom told me I may be pregnant... I seriously doubted it. Anyway, tested yesterday and I got a positive! Very faint line but definitely there. Today I had a negative. Same brand test, also taken with fmu. I have an appointment Friday afternoon for a blood test but I'm a little worried. Is it really just too soon to get a positive on every test? Maybe my hcg is still so low that maybe even my fmu was diluted? (I had a lot of pee, more than normal bc I had a lot of water yesterday)... Hoping and praying for a baby!