Clearblue tests help....?

ok so i recently switched to the yasmin pill stopped for my break and havent had my period for over 3 weeks now, so decided to do a preg test got 2 clearblue plus ones and both came up with a very very thin faint blue line so started to panic a bit so got superdrug brand one and did two and nothing so hopefully i am not preg just need to know what this means but surely i cant be if the superdrug ones are negative please help i dont know what to think ahhh.....