104lbs, 18.1 bmi. usually regular periods, had unprotected sex when 1 day late. took planb 18 hours later. now 4 days late, no spottin. preg? anorexic?

Too soon to tell: Cycle lengths from 21-35 days are considered normal. If your cycle isn't regular (perhaps due to low bmi), then it's hard to know when your fertile time was, but since you took the plan b, it's unlikely that you conceived. Unfortunately, you won't know the outcome until approximately 2 weeks after the last unprotected sex because that's the earliest that a pregnancy test will be accurate.