BFP then BFN

Feeling a little disheartened today :(
AF 3 days late (due on Monday) 
Monday morning (fmu) I took a boots own test and got a faint line within 2 minutes at most, I was so excited!
I then got a vvvvfl on a first response on Tuesday morning (fmu) but this morning I took another first response and all that came up was an indent :/ any ideas?!
I've had no bleeding which gives me hope that it's not a chemical pregnancy.
Photos in order of days.