Positive clearblue digital then negative boots digital

Charlotte
Has this happened to anyone else?
I was going to call the doctors today to make an appointment after getting a positive with a clearblue digital at the weekend (see pic below).
But took a boots digital test this morning (3 days later) and got a 'not pregnant'. Bought a superdrug dye test and used smu and getting a very faint line but thought it would be stronger after a few days. I'm 5 days late for my period. 
Still no period, could this be the start of a chemical pregnancy? :( 

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