do i retest?

so i had sex on november 10th and took a plan b im on the pill and never miss it but the condom was being really weird and slipping down so we did it just to be safe a week goes by we have sex again with bc pill condom and everything but the condom falls off and gets stuck half way in and half way out and i’m pretty sure some of his ejaculation leaked out near my vulva. i’ve been late for 15 days and took a test on the 21st and it was negative should i be worried and retest now or is the negative accurate enough?