I am 18yrs old, 60kg and 5ft 3in.  I am very athletic and generally no health problems.  Since March I have been experiencing an on/off pain behind my navel which is causing a lot of discomfort and I would describe it as a cramping/gnawing pain.  Sometimes the pain is sharp and travels up into my chest area.  I have already had my appendix removed and I am not pregnant.  I also experience nausea and occasional dizziness.  I have taken probiotic with peppermint oil capsules, and more recently omeprazole and sometimes take ibroprufen for tension headaches.  I am currently taking my A2 exams, so understandably I am stressed and reluctant to spent valuable time in doctors surgeries or A&E.  I was hoping for some reassurance.
Be assured, All the history you have noted about the pain and cramps behind the naval that travels up to your chest is more suggestive of IBS- Irritable Bowel Syn Chat Doctor.  Continue the probiotics as it helps a bit. Since you are stressed out, you need to vent out, get relaxed by yoga, meditation or take a few medicines for relaxation, your pain will also go.