Brown discharge

Hi ladies! So I’m late 18 days late.. for the past 5-6 days I’ve been having brown discharge . It wasn’t too much, but for the past days it came out also when I wiped along with very little pinkish discharge at times. Took a pregnancy test yesterday morning & it came out negative. I’m 36 never been pregnant nor have been able to conceive. I normally get my period , but there’s been a few instances (rare) when I skipped a month.. but this is the first time I get this discharge instead of period. I normally get brown discharge more like dark red when I’m finishing my period. Help!! Breaking my head what it could be :/