What is causing this pain?

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I've had a grinding pain in right lower quadrant of abdomen (between hip and naval just below naval height) for years.  It was one and off but now it is more regular and a daily occurrence at least.  It lasts for hours.  The area always feels tender.  On top of this, the area above this and leading up to just under my ribs has started to swell up massively about an hour after I eat.  It gets so bad I feel like I have a blown up balloon in there that presses under my ribs on the right-hand side and extends down to the area where I've been getting so much pain.

My GP diagnosed IBS.  I've been to gastro three times in all.  First time (ca 30 years ago?), had barium x-ray and he diagnosed IBS.  Second time, so years later but still with intense pain happening regularly, had sigmoidoscopy with biopsies, and still same conclusion.  The pain gets really bad, grinds and grinds and refers down through my hip to my knee.  It prevents me from doing things, focusing, getting exercise, because it is so uncomfortable.  All I want to do is to get away from it.

I take alverine citrate forte for the 'spasms' but although sometimes it seems to help a bit, mostly it seems to do nothing.  After struggling with paracetemol and antispasmodics for years with little pain relief, I now have co-dydramol which is a little better.  It still doesn't remove the pain but reduces it more than paracetemol alone.  I feel this is a high level of pain to have to live with all the time.  I use the medication sparingly as I'm aware it is addictive and also it causes constipation.

Just recently (yesterday) had gastroscopy and colonoscopy.  They diagnosed hiatus hernia and diverticulii in the local and distal sigmoid (which I understand is on the left side of the abdomen?).  I get no pain on the left whatsoever so this is no help to me.  I am awaiting biopsy results but preliminary report is that things look 'healthy'.  Since the test, I have had the grinding pain back again.  

I have had an abdominal ultra-sound scan recently too to rule out ovarian problems due to the swelling.  My GP assessed me when I had some swelling and said he noticed a 'fullness'.  The swelling is all on the right-hand side of the abdomen and it feels like when I was pregnant and the baby pushed her feet up towards my stomach; it's that kind of pressure.

I'm at a loss really.  If this is IBS, why is it so severe?  Why has this bloating and swelling symptom developed higher up?  When I have it, I feel like I can't bend over because I would be trying to crush a balloon in there, such is the pressure.

Could there be any other explanation for this pain that can't be detected by ultra-sound scans, the tests mentioned above and numerous blood tests?  I feel I can't go on like this if my life is going to be nothing but struggling through pain.  I already have fibromyalgia and I'm extremely sensitive to cold.  Fortunately, an antidepressant really does help with the fibro pain but it can't seem to touch the abdominal pain.  I don't want to pester my doctor if there's nothing that can be done, but life is rubbish.  I can't do the things I want to do because of constant pain.  I don't think life should be lived like this.  I do have Ehlers-Danlos syndrome (hypermobility type).  I don't know if this has any relevance to the pain.

I would really appreciate any thoughts on this as I am truly despairing of ever having any kind of life.

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I also forgot to mention that when I get the pain badly, it refers down through my right hip down my right leg to my knee.  

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Oh, I forgot to mention, I'm also low in iron stores, hence the recent investigation.  I really feel like it is best to starve rather than keep having this pain.