What Does Intermittent Positive Results In HPT Indicate?

Hi, I am 19 years old and I took a pregnancy test about 2 and a half weeks ago and it said not pregnant, but then last night I took another one and it said pregnant, after i took that one at around 5:15 pm I took another one around 11:00pm the same night and it said not pregnant so now I am confused at what I should think or do?