It's not attached to anything? Anxiety is irrational thinking, but it's usually attached to something. That something might be, say, driving despite never having been bothered about it ever before -- anxiety does seem to come out of nowhere for most of us -- but you seem to be saying it's not attached to any thoughts or activities. Is that true?
Paxiled has some good points. One of the challenges of anxiety is to try and find the triggers. I agree about seeing your doctor for a physical; never hurts to make sure all cylinders are firing as they should be.
One of my weirdest anxiety triggers is not being able to sleep. My fatigue feeds it, and it can quickly become a cycle. Are you exercising? How are your eating/drinking water habits? Do you drink a lot of caffeine? All of these things can impact anxiety levels.
Hang in there! Call your doctor; a co-pay can be a small price to pay for peace of mind.
This is called free-floating anxiety and it's a common symptom of anxiety and especially anxiety disorders such as GAD. The best thing to do is rather than search for a cause for your anxiety, try to focus on more productive thoughts and actions. Let the anxiety be there without taking over your life.