IBS is an increasingly common problem in the UK, and is usually characterised by a collection of symptoms, which can include stomach pain, diarrhoea and / or constipation.
The causes of IBS are unknown (although food intolerance, infection or bacteria, lifestyle and emotional stress are thought to be linked in some way and may trigger many cases).
Following examination and investigation, it may be that no physical reason can be found for this often vague collection of symptoms, and IBS is the only diagnosis that can be given. The symptoms can also fluctuate; sometimes you may be really affected on a daily basis, whilst at other times you experience no symptoms at all.
You can find out more about IBS at NHS Choices.