So physically it can be a body memory (fear coming up) but many times it is when my heart rate will pick up on it's own or my thyroid messing up my "electricity" (heart beats, etc) My grandfather had a similar condition and told me as I was growing up that it was okay and not to worry because people with anxiety live longer. Probably not true, but he lived until his mid 90s... So whatever I'll go with it!
Emotionally it comes up with memories, PTSD, thoughts, etc.
No matter how my anxiety starts, it pretty much feels the same. My experience is so hard to explain... I just feel "fast" as if I have little balls of energy running all around me, my personal space gets huge (anyone within even 20ft of me I just get more tense) and any one I know around me I am hyper attuned to them and very internally tentative. My heart rate is quick and my thoughts are very fast and succinct (this works great for when I'm under pressure - I can't write unless I'm in a state of at least some anxiety). If it's mild I will feel a restless discomfort and a need to move/walk (this is usually as far as my physical anxiety will go). Emotionally caused I will start to fear for my life, need to be alone, shake, my thoughts start streaming down like a waterfall of 5 things at once. I feel like I'm playing hide and go seek and the seeker is walking right past where I am.
I didn't realize how much it impacts my life until I took inventory of some of my habits in therapy. I go out of my way to avoid a need for phone conversation, I have nightmares or night terrors and constsntly use my pets and their reactions to figure out if things ARE unsafe or I'm just feeling unsafe... Worst is I'm not really involved in my life - sometimes my mind is elsewhere or it causes me to dissociate or... I'm feeling so afraid and "fast" that I'm calming myself on the inside so I can maintain the outside and I'm therefore missing everything
Sometimes suicidal feelings or urges will come up, I'll get insomnia, I don't recognize myself in the mirror and get very body conscious, I tap my chest. It prevents me at times from shopping in the same grocery isle as someone else, stop me from entering stores with no customers inside or loud music, prevents me from asking for what I want, keeps me in bad situations, blocks confidence and worth... It's no fun at all. I have physics symptoms too like site muscles, extreme fatigue, etc.I've learned to take the energy and move it in to other things sometimes. It's always in the background though. I get triggered when my body relaxes sometimes (even when I'm falling asleep so I take medication for that, too). I'm not sure what I would feel like if I didn't have at least a 2/10 level of anxiety on all the time. I think I manage it well, it's really only the bad storms or full panic attacks that I still struggle with.
Not sure if that helps... People often describe me as a nervous cat. Untrusting, skittish, but sometimes relaxed... But cats are almost always sleeping "with one eye open" and ready to scurry at any time.