So, I think the most common one is stress. Often times I realise that my forehead is feeling tight, and other telltale signs arise to show that in fact I have been feeling stress, but haven't felt it directly.
Another common one is anger, where it is only after my actions take place that I realise the base motivation was anger.
Yes, I experience this quite frequently. Supposedly the body experiences emotion in the form of several stages of physiological and mental recognition. If you're distracted or lost in thought, you might not recognize and name the emotion. Hence suddenly realizing that you're clenching your jaws, or furrowing your brow or saying things out of anger.
I don't think it's a big deal, but it is a good way to end up sitting around stressed, which isn't good for your health. As others mentioned, just checking up on your body every once in a while and identifying what's causing the emotion is helpful
"There's no need to be embarrassed about it, Mr. Spock. It happens to the birds and the bees!"
Stress manifests itself for me with need for a lot more sleep, teeth grinding at night, tight jaw, gripping the steering wheel with a death grip and one shoulder going all sore and wonky, and yet much of the time when I experience those symptoms I am not aware that I am feeling stressed. Sometimes when I come home and start talking about my day, thinking that it wasn't too bad, I find myself acting surprisingly emotional about something that really had bothered me, but that hadn't registered how much it did until I started telling about it. That rarely happens with anger, although there have been scattered occasions that I thought I was able to deal with something and when someone pushes their luck even further I end up saying what I had intended to say to them, but at a less opportune time than I normally would have and much more bluntly. Sometimes when I talk about something that I am sad or embarrassed about, I am surprised to find myself being on the verge of tears when I thought that I as doing fine. These occasions aren't often, but they always surprise me (and the other person!) when they happen.