SS17, what part confuses you?
It makes sense now! Yay.
You are close in that first paragraph. The INFJ doesn't just forget about the other incidents. They are frustrated at the time, but don't want to be hasty and overreact. They are trying to explain when they finally do explode that they have done everything they can to deal with this problem on their own and that the other person has overloaded them with enough things this is no longer possible. It is not just about one seemingly tiny incident. They hate being seen as oversensitive or wimpy. They are usually able to handle quite a lot and they also don't prefer to become emotional and cry in front of other people. This is usually the point at which some well-meaning T explains that they are being oversensitive and making a big deal out of something that isn't. (Which adds gasoline to the fire, even while they are frantically shovelling dirt and beating wet towels over it internally in efforts to quell it as soon as possible). The incident they snap about is just the straw that broke the camel's back. They actually hate this feeling and are doing their best to bleed off enough emotion that they can once again be as objective as possible and return to a calm way of dealing with things. Because of not wanting to be seen as difficult or wimpy, often INFJs don't completely finish expressing all of what has been rankling, which is why the topic gets revisited several days after you thought you got it all worked out with them.
The description of the shy laugh later in your quote is her trying desperately to get back to normal, but then thinking about the things that are still not fixed that are bothering her. She's doing her best not to be that way and the silence certainly isn't for dramatic effect.
Okay, that totally explains everything. Now I understand that she's actually doing a lot of this to not seem overemotional, and that it's not about the last thing that made her angry--it's about previous mistakes as well. It doesn't seem confusing at all now. Wow, your description of the INFJ trying so hard to make things better again has really made me respect and appreciate INFJs even more. Thanks!