The doctor prescribed me lactulose for constipation. She might have forgotten that I mentioned I was lactose intolerant. Should I still use lactulose?

Possibly. Lactulose is lactose based. Some people who have mild lactose intolerance do not have problems with lactulose. Others get bloating. If it is important for you to have the lactulose, a lactase senzyme supplement may make it more tolerable. Call your doctor and ask.