Hi
Move aside and let the professional through .
Ok, so we have ruled out an intolerannce to alcohol itself if you are OK with wine and spirits
So it will be either Yeast or Gluten, or very very possibly fish
Yeast , Real Ale is crammed with live yeast cells , thousands of the little blighters in every ml (millilitre ) and they are possibly causing you an issue with inflation , quickly followed by deflation. Which is not uncommon. It is a shame as you are going to have to search out either filtered craft keg beer which is becoming more widespread, and /or filtered Lager
Or you have developed a Gluten intolerance which you can check at home with a bowl full of pasta , a few slices of bread etc,
Or you are having a reaction to Isinglass , which is the macerated swim bladders of fish from SE Asia and is used to coagulate the yeast and it falls ( mostly ) out of susp ension in a lump intertwined with the Isinglass, however this is extreamly rare and as itusually settled in the bottom of the cask it does not have to be listed as an allergen
Mainstream big boys Lager is mostly 2x or 3x filtered before pastuerising to stop living stuff from reproducing . The final filtration is usually tight enough to remove 100% of yeast cells So if you are OK after drinking Carling then its a yeast thing
You could do a simple experiment. Buy 1 or 2 BCA’s Bottled Conditioned Ales, store them inverted for a week after properly shaking them , then pour into a glass, yeasy bottoms ( which will now be at the top , and everything . Farty Pants= Yeast intolerance as the yeast in a BCA should be alive and kicking .