In my location (and pretty much anywhere really, because the symptoms of dissociation don't change from location to location, even if the understanding of it does) this can be part of a dissociative problem. Strong emotions that seem to go from an intensity of 10 to nothing at all are recognized by leading international researchers as being part and parcel of dissociative symptomology. Same thing for strong emotions that 'come out of nowhere'.
This happens to us a lot too. Like we have one little who has very intense fear, and whenever she makes an appearance we can go from 100% okay to a state of absolute terror in zero seconds flat. And then in an instant she can disappear and we just pick ourselves up and get on with whatever the heck we were doing. But for that time when she is out we are (she is) a freakin mess.
It kinda makes sense that you aren't familiar enough with those emotions to name them in you, since you don't seem to experience them as integrated emotion.