Who should I see if I have serious constipation?

Gastroenterologist. A GI specialist is an expert on all issues of the gut.
Start with reg Dr. Your regular dr can evaluate your issues & decide whether lifestyle/diet or other simple modifications can help. Plan on providing a history including diet/meds/supplements& emptying practices.Initial evaluation could result in a rx plan/tests or referral. I encourage u to arrange this soon as delays often slow recovery or detection of significant problems.