There are no tests for most kinds of food intolerance. All a specialist can do is diagnose based on the history - what happens when you eat certain things.

What they will usually do is try an elimination diet, where you remove the basic foods most likely to cause a problem, and introduce them again one by one until the symptoms recur, whereupon you know what does it. You can organize this yourself. In this case the most likely candidates are chocolate or dairy foods. (There are many different kinds of dairy intolerance - people can react to lactose, dairy fats or any of many different proteins, and all of those have different implications for what you can safely eat). It isn't difficult to manage your diet to exclude both of those and experiment with them separately.

But for an elimination diet to produce meaningful answers, the exclusion has to be total. When excluding dairy you have to exclude it in ALL processed forms. Desn't matter how little, what name it goes under or whether you can see or taste it. If any molecule of it came out of a cow's tit, avoid it.

Once you've tried the elimination diet yourself you have much more data to present a specialist with, if you still need to see one.

I am guessing that when you write "This seems to happen every time I have it", "it" means chocolate, not strawberry oil.

It's getting increasingly difficult to find dairy-free chocolate in the UK, but it can be done.

Finding food intolerance specialists is difficult.