Lower abdominal pain

If eating low calorie for the occasional day made you gain, 5:2 wouldn't work for so many people.

What are you eating? That might be just as important as how much, especially as you have IBS and a fluctuating appetite.
Are you regular? Because if your digestive system is sluggish and bunged up, it's not going to help your appetite, and you're going to weigh more if you're filling up but not emptying out properly.

Gut flora is responsible for a huge amount of appetite. Healthy flora thrive on a healthy diet. It might not be the whole solution, but there's no cons to making sure you're limiting the processed foods and getting plenty of veg (unless you're in kidney failure, maybe).