Do a slow burn
Definition 1 of 2
Pro Tip 1/3Subtle Intensity
It expresses building anger discreetly, without immediate confrontation. 
After the meeting's passive-aggressive comments, I was doing a slow burn.
Pro Tip 2/3Extended Time
The anger doesn't peak instantly; it builds over time. 
Throughout the chess match, my opponent's smugness had me doing a slow burn.
Pro Tip 3/3Controlled Reaction
The person remains composed outwardly, while anger grows within. 
She kept her smile, but inside, she did a slow burn over the unfair criticism.