I was like that also.
I'd go for days and days and feel great, then wake up one day and feel like crap.
I swear anxiety has a life all it's own and it'll do anything to freak up out!!
But I did learn that when you do have those bad days you just have to go with it, don't look at it as a set back, it's just a little bump in the road that you can and will get over.