It could also be that because you have been over and under eating, your metabolism is a bit screwed up, and your body has gone into "starvation mode" which happens when you yo-yo diet. You eat not enough, and so your body thinks you are fasting, or that there is no food, so it slows your metabolism down and then when you DO eat normally, or more than you did, the weight goes on far quicker than normal.
However this doesn't explain the pain. Your Doc is clearly a waste of time- you should phone your surgery and ask to see a different doctor as you would like a second opinion. If the Doc is as unhelpful to all his patients, the surgery will be used to patients complaining about him.
If they query this, say that you are not happy with him and can't afford to go private. You are paying your NI so are entitled to health care. At the moment you are receiving no care whatsoever.
You have to stamp your feet. If they can't find anything wrong with your stomach, insist to be referred to a gastroenterologist at the hospital. Rather than wait 3 months for an appointment you can tell the Doc that you want a private appointment. He will then write a letter requesting one on your behalf, to the specialist. This will cost you maybe �100-�250 but it does mean you can queue jump. THEN when you see the specialist, tell him that for any further treatment/appointments, you want to go back to NHS, so it won't cost you any more. I have done this a number of times just to get stuff sorted.
Good luck- let us know how you get on.