In my experience, anxiety and panic can seem to 'come out of the blue,' however there are many different reasons we experience it. You said you have not been feeling stressed but have you had a lifestyle change like less sleep, different eating habits, etc? Also, with me, I sometimes don't really realize that I am stressed until someone points it out to me. Also, it is quite common for people to initially experience panic and anxiety for the first time in their twenties...that is when I experienced my first panic attack.
Have you ever had a history of panic or anxiety before this experience?
Sometimes being ill like with a uti can also affect your mental state a bit like getting more anxious if it remains I suggest you talk to your gp about it get some help.Amxiety doesnt always have a direct reason or cause.
You have nothing that you are worried about? I've been feeling like this lately too, I found that sometimes it can be completely subconcious or it could be the way you are eating or not eating enough! Try eating al healthy and lots of water, this always helps me!