The first thing that comes to mind is that you might have milk intolerance. That is, my theory is worth considering only if you tell me that you've been trying milk
chocolate, rather than dark
Many people, as they get older, lose the ability to produce milk-digesting enzymes. Plain cocoa has no milk in it, and you say you add it to water rather than milk. Most prepared chocolate, whether chocolate chips or bars or prepared desserts, have milk in them. Have you read the labels carefully to check whether there is milk in any of the chocolate that is giving you problems?
If you decide to try carob, it is a completely different plant from chocolate. Some people find that it tastes exactly like chocolate, and others (myself included) find that it tastes quite different. But it can be a good and tasty substitute.
Good luck in tracking down your food reactions.