Irritable bowel syndrome (IBS) caused by food intolerance.
It's normally caused by anti-biotics or a major physical change in your life (pregnancy, car accident, etc), or both.
The most common foods that cause it are dairy/milk (lactose) and wheat/ gluten. Food preservatives, and ingredients that are numbers will also cause IBS. Onion and onion relatives (leek, shallots, spring onions and garlic) can also cause problems. Eggs can cause problems too.
Check all ingredients and if they contain dairy, wheat, gluten or numbers, avoid them for a couple of weeks and see how your insides feel.
Have meat and veg for a couple of weeks. No bread, cereals or cows milk. If you need cereals, try gluten free for a few weeks.
Be very careful about reading labels because most packaged foods have wheat/ gluten in. This includes things like icecream, chocolate, cornflakes, ricebubbes and corn flour. Yes, most brands of corn flour are actually made from wheat flour. It's false labelling but consumer protection doesn't care.
If your insides feel better after a couple of weeks on a basic meat/ veg diet, try adding one of the bad foods (glass of milk) and see how you feel over the next 24 hours. If it's fine then have a glass of milk each day for 2 weeks and see how you go. If it's still fine then add another food like bread and see how you feel. Continue doing this until you find the food that is triggering your IBS.