Is it still possible to be pregnant after 1 positive and 3 negatives?

I done a test on the 14th and it came out as a very faint positive and then i done 2 on the 16th and 1 on the 17th and they 3 both came out negative. Could i still be pregnant? or could that of been a duff test? i will test again soon though just to make sure before i go to the doctors