Have you had any unprotected sex in living memory? Some STDs can cause abdominal pain and problems with periods years after you have caught them. You can get free STD checks from your GP or GUM clinic. The GUM clinic don't pass any information on to your GP without your consent so if you want to keep it off your medical records, that's the place to go.

Try keeping a pain diary of when the pain starts, when it gets worse, what sort of pain it is (e.g. sharp, dull, ache, intermittent) and when it stops. Record what you were doing at the time, if you had eaten anything etc. It may help narrow down the possibilities, especially with food intolerance related diseases.