My Husband Has Just Completed His 4th Infusion Treatment And

My husband has just completed his 4th Infusion treatment and has begun to have severe back pain. He had issues before with his back but nothing like this. Of course he cannot take Motrin due to his Chrohns Disease and he has had a Total Colectomy with a J-pouch (in 2002). Is this normal or should he contact his doctor?