Was on the pill dilligently but missed last active pill. took two plan b on the same day right after that. period late 4 days, hpt negative. could the delay be due to the double plan b dose??

Absolutely.: When on oral contraceptives, periods are actually withdrawal bleeding - your endometrial lining bleeds because you're not taking the hormone those days. Plan B is a large dose of the same sorts of hormones, and a delayed period is not unusual when taking it near the end of a pill pack's active weeks. Check home pregnancy test 2-3 weeks after intercourse for most reliable timing.

Likely: The delay in your period is likely due to plan B. You may repeat the pregnancy test in 7-10 days, unless you get a period by then. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex if you have sex.