Yeah, it's possible to be constipated with Crohn's. But that said, it is possible for severe constipation to cause as bad pain as you described.
I think you should find another GP, not 'cos I think it's very likely you have Crohn's but the constipation ought to be investigated further if it's that bad. Google colonic inertia: it's a real thing and more Fybogel definitely will not help. Find a more sympathetic GP who will refer you to a GI.
Dx Crohn's in June 2000. (Yay )
Tried: 5-ASAs, azathioprine, 6MP, Remicade, methotrexate, Humira, diets.
1st surgery 20/2/13 - subtotal colectomy with end ileostomy.
2nd surgery 10/7/15 - ileorectal anastomosis. Stoma reversed and ileum connected to the rectum.
Current status: Chronic flare. Do I have any other kind?
Current meds: 50mg 6MP; Entyvio (started 3/11/16)