What Causes Brown Vaginal Spotting And Back Pain After Taking I-pill?

I took meprate tab for 3 days since Jan 4-7..My doc sd i will gt my period aftr 10 days..no result on 17-19..had unprotctd sex on 19th..took ipill on 20th..Lite spotting of brown blood yestrday n today..im hvng severe cramps...n unbearble back pain since last week..did i get my date or is it due to ipill..im confused!!