No specific advice except it will pass!
That inability to pinpoint what is causing the anxiety exactly always sucks. I had one of those days just the other week, and it was horrible. Stuff to help work differently for people, try doing something you like (go for a walk, get that really good sandwich, pick up a nice chocolate bar later), try writing down how you feel and throw it away if you need to after.
Hopefully even just venting here helps, it is happening and you will get through it, we are all here for you