What if I just took one dose of Plan B will will it work?

Plan B can cause all sorts of hormonal side effects, & back ache is one of them. However, if the pain is very sharp, then I would get medical advice from a doctor. Although it's unlikely, ectopic pregnancy is something to look out for, when pain is very sharp. If it persists, see a doctor.

In terms of whether one dose will work, Plan B is not as effective as regular hormonal birth control, it's simply worth a try in an emergency. If you are ovulating, it may not work. The only emergency contraceptive pill proven can prevent pregnancy during the 24 hours around ovulation, is Ella (Ulipristal Acetate), & even that pill is not guaranteed to work.