42nd Day And Still No Period. I Usually Got It

42nd day and still no period. I usually got it within 29th-37th day. the longest was 39 days and it happened only once.
my husband and I had unsupported sex 10 days after my first day of period. 2 weeks ago I had a light brownish spotting. today is my 42nd day and still don't have my period. i tried a pregnancy test today and got NEGATIVE result.
is it possible that the result wasn't right and take another test or should I wait consult emdiately my doctor?