Magesium chemical reaction in paper cup

A family member just took some magnesium citrate powder in a glass of water, which she does every night without issue. Tonight she put it in a small paper dixie cup. She drank it before she realized the cup was heating up. The magnesium citrate was having a chemical reaction with something in the paper of the cup. I have no idea what it would be, nor if it is an issue that she ingested this. We called poison control, but they couldn't tell me anything. Just as a precaution I had her take some activated charcoal. Does anyone have a clue as to what chemical would make magnesium citrate heat up and if we should be concerned? Tweet