I'd say that partly the bottling up of my feelings comes because I am trying to understand them before I allow myself to feel them (Ni-Ti loop).
This, totally. Ni-Ti loops can look a lot like Ti-inferior Fe. If I feel anything I am not sure is in proportion to the situation, an appropriate response, irrational or impractical, I try to shut it down. (This is speaking more for the past, as I am a lot more in touch with my emotions now.) My response is to hyper-analyze my response, identify all the underlying beliefs, and evaluate whether or not they are true, then tell myself what is true. It actually helps a lot, but incorporating a more feeling-friendly approach is more effective.
And then partly because I'm all Fe in the world, I don't want others to feel bad so sometimes I cover up my bad feelings, and that can equate to not allowing myself to feel them.
For me it doesn't have anything to do with how other people feel directly; I am indirectly very concerned that I will alienate people or make them very angry at me and then they will be hostile to me. Also I tend to assume I'm overreacting, so I usually go over and over the situation and get another perspective on it before I'm sure I am justified in feeling how I do.
I have read that INFJs tend to feel other people's emotions, but have real trouble with their own...
For me I have trouble sometimes identifying how much of what I feel is mine and how much is someone else's. It's easier to detach from a situation if I know I am just absorbing someone's mood. Also I have higher standards for myself than I do for other people, so that means standards of practicality, rationality, logic, and maturity (which I have traditionally and erroneously seen as being in conflict with emotion). If someone else makes a mistake I'm more likely to be indulgent.
When your feelings explode, what does it look like?
I feel like I am full of righteous anger or indignation, and I can't be nice any longer. I feel there is injustice going on which I am going to fight for. I have something to say and it needs to be said (or so I think), so I say it. Normally I try to be objective and rational, but when my feelings explode I feel that the truth is not objective, if that makes any sense, which it probably doesn't.