What Causes Heavy Vaginal Bleeding?

My wife is having heavy vaginal bleeding 4 days after taking Levonorgestrel ECP which she took one hour after the intercourse which caused her so much pain under her belly & weakness in walking. She had her last period bleeding one day before having intercourse. My question is that is this bleeding a side effect or is it normal? Also how to cure it & get recovered from the condition she's having & when she would have her next period?Thanks