If you have IBS then it's to be expected that SW will make you constipated. The plan is full of high fibre foods, fruits, vegetables and things that ferment in your bowels causing constipation and gas/bloating.
I have the same issue and was referred to a dietitian who told me to completely avoid the brassica family of vegetables (eg cabbage, cauliflower, broccoli), don't eat baked beans, chocolate isn't good, don't eat the skin on fruit. She said it's important to eat fruit and veg but you should be eating more vegetables than fruit. Drink LOTS of water and eat regularly, no more than 4 hours without eating. Watch your fibre intake as many people with constipation and IBS have to eat a low fibre diet (so fibogel is a big no no!)
I'm re-starting SW and I have to tailor it to my IBS and what the dietitian said or I'd be awfully constipated and bloated.