Day 36, no period

I get my period regularly, about 30-33 day cycle. The only time I was late was in Dec 2019 when I was very sick. I was on Clomid in March but got a regular period after that. Due to covid, did not have a 2nd round. I usually ovulate around day 17/18. I did one first response pregnancy test on 18 dpo and it was negative. Any ideas of why I could be late? Any chance I can still be pregnant? I have been trying for 1.5 years.