Gagging while eating ?

Does somebody have an idea why I have been gagging while eating? I have been sick for the last couple months due to drug withdrawal. I experienced a lot of nausea then but it is almost completely gone by now. My appetite has mostly returned. But while I am in the middle of a meal I gag. I do not overeat at all. In fact I would say I probably don't eat enough. I do not allow myself to vomit but I am having trouble finishing my meals due to gagging; it is starting to hurt & upset my stomach; even though the gagging is very brief it happens pretty much every meal. I don't want advice on how to stop gagging, I want to know what is causing this. Thanks!