When a word is given as uncountable in a dictionary, it doesn't mean that it can never be used countably in any sentence. Countable/uncountable are general descriptions of the normal behaviour of the word. I would imagine that the writer uses the article to suggest that the emotion that they felt wasn't particularly satisfying, possibly a bit strange or artificial. When a feeling is related to a particular time or set of circumstances, you may find uncountable feelings taking an article- I felt a great happiness when I heard they had been rescued.
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