I menstruated may 5th-8th. husband released inside of me on the 13th. had spotting on the 18th... could this be implantation bleeding?

Unlikley.: You ovulate 14 days BEFORE your next period starts (there is about 48 hours around this time when you can conceive); you can only tell these things after the fact. I assume the dates you gave were progressing forward in time. If you have a normal 28 day cycle, then it is not likely implantation bleeding, as implantation occurs a few days after fertilization, which occurs after ovulation. See DOC.