Samething happened to me. Every test would come out negative until i was 2 months pregnant it came out positive. It sometimes happen to some girls but if you do a blood test it will show quicker. Or you can just wait a week or two and it will show a positve or negative
make sure you use first morning wee when you test, treat yourself as pregnant until either your period comes or you get a positive, or you could go and get a blood test done with your doc :)