Saving up feelings and complaints and then "dumping" them during an argument is known as gunnysacking.

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Gunnysacking is a communication pattern where someone avoids expressing their feelings or complaints during the course of a relationship, instead saving them up until an argument arises. At that point, they "dump" all of their pent-up emotions on their partner at once, creating a stressful and emotionally charged situation.

Saving Up Feelings and Complaints and Then "Dumping" Them During