Severe constipation/Metamucil

So I've been severely consipated for over a week, I went to the hospital due to back side pain the other day and they suggested that's what it as so I thought okay I'll try mostly everything to have a proper bowel movement before I go back and see if my pain is for something else.
They gave me Movicol and Actilax (laxatives) and they are barely working.
My question is, is there anyone taking multimucil for constipation if so would you recommend that I try that instead of the laxative drinks ? Thanks