IBS is a very loose 'diagnosis', that basically just means your gut is irritated - there are a large symptoms that get lumped under the heading of IBS, so, labelling your symptoms IBS doesn't really get you any further forward. In your specific case it's not surprising that you get that pain because constipation (and evacuation of a constipated bowel) is extremely uncomfortable and puts strain on all the tissues around the abdomen, including the spine. If you have been blocked up for long enough it's very common to get a bout of diarrhoea as the unhappy gut wall is irritated by the arrival of another meal. The diarrhoea in this case is due to the constipation.
Working on easing the constipation is the most important thing to get rid of these symptoms. Check your diet for lots of lovely cooked veg, stewed fruit with cinnamon and ginger, and nothing white such as white flour, bread, pasta or rice. Drink heaps of warm water but not within half an hour of food. Avoid caffeine as much as possible.
You may need to take a laxative such as Linoforce initially, but the dietary improvements usually keep things going after that, unless there's another factor at play such as medication that causes constipation as a side effect.