Help confused !!! 1 faint positive and 2 negs

Hi I'm wondering I don't a pregnancy test mid afternoon At work...30 mins later I looked at it and it clearly shown a faint positive line.

i done another test that evening...negative. 

And one mid afternoon next day...negative. 

my perioda are not regular but I would say I've got about another week to go until my period is due. 

Ehat does this mean ?? Could I be pregnant ??