I once had this experience before whereby my period was 12 days late but not pregnant. I thought i was bcz my period is alwaes on time and regular. Plus we weren't 'trying' to conceive at that time. So I am not sure why that happened as i didnt go and get checked as my period was normal aft that. Maybe you can try to test again in a weeks time as some ppl mite show negative but later on get positive.
This can happen due to Irregular period or pregnancy . it is better to do another pregnancy test after few days.