Covid test

My husband had a slight fever and a headache for only one day a week and a half ago. 11/7. He went and got tested for covid the following day. Last Tuesday his results was "detected". It has been been a week and a half and his job wanted him to get retested before going back to work. He got retested this Tuesday and the results was "not detected". The doctor said the results was definitely going to be positive again since it had only been a week and a half. Doctor said if it shows negative then his first test was probably a contaminated sample or something. Doctor said the first time he didn't think my husband even had covid because he didn't have any symptoms, just a very slight fever for one day. And a headache which my husband thinks was a normal headache because he gets them often. So what could have happened? How could he have had a positive test and then negative barely over a week later?