Help???

I’m 2 days late for my period (which I know isn’t long), but my husband & I had sex during my fertile window this month with no BC. I always start my period on time or early so this is very weird for me. I got a pregnancy test from the dollar store (the dollar ones) and took one and it was definitely negative. Has anyone got false negatives from the DS brand tests before? I also took it late in the day. I’m about 15 DPO. Any advice????