Thinking im pregnant?

Patient: I am not pregnant, i am on the pill, had periods and have tested negative on 2 tests. Yet i cant get the idea that im pregnant out of my head, i do not have any other signs of pregnancy and deep down i know im 100% not, i havent even had sex for 2-3months atleast! but i cant escape the feeling that i am, i keep wasting endless money on tests! Why do i keep thinking this??