Sudden changes in Lortab effectivess

I have been taking lortabs for lower back pain (Ankylosing Spondylitis) & terrible menstrual cramps for about a year now. They used to take care of the pain & give me a euphoric feeling and I could function better while taking them. All of a sudden they have started to make me feel very drowsy and weak. Why have they all of a sudden made me feel different & will that drowsiness/weakness ever go away?

Thank you