Yes, this seems to be the story of my life the past six months! I find that others experience the same thing, too. I hope that you can take some comfort knowing you are far from alone in this regard.
I agree with KC above that there could be a trigger you have yet to realize, but I'd also take it a step further to say that anticipation of the undesirable symptoms that come along with anxiety (depression, etc.) can be enough to make one anxious. For me, this has been a very important realization on my road to recovery. What I used to think of as "no reason" for my anxiety I now realize is just fear of feeling afraid, depressed, and all the symptoms that come along with it.
What really helps me cope is to examine or "shine a light" on each and every symptom as they come: the depression, the brain fog, the jitters, the back aches, and so on. I say aloud what each is and how it's making me feel. Then, most importantly (and most difficult!) instead of tensing up to fight the anxiety or shrinking away from it in fear (both of which only feed the fire) I try to accept the symptoms and "float" through them – yes, they're here, yes, they're miserable, but no, they cannot truly hurt me, because they are just a product of berserk adrenal glands.
I used to find myself getting really down about my anxiety and throwing my hands up in the air in frustration because I was "losing the battle" and scared I'd be like this forever --- but now, I realize that treating it like a battle at all was keeping me from recovery. Acceptance teaches our nerves that it's okay and we can gradually stop the fear-adrenaline-fear cycle which is, effectively, anxiety for "no reason."
Have you read any books about anxiety? Lots of what I wrote above stems from this great book called "Hope and Help for Your Nerves" which I learned about on these very boards. I just started it the other day, but it's already helped me tremendously. It has very insightful advice and I appreciate that the tone is not overly soothing or patronizing (which is all too common in self-help literature.) I got the Kindle version and I'm really happy with it as it syncs my highlighted passages between my phone, laptop, etc. so I can just whip out my iPhone when my anxiety is getting out of control and re-read some choice selections.
Hope this helps a bit!