I feel so angry and so cheated because this is supposed to be a time for me to celebrate.
This is one of the big problems with anxiety - we have expectations and then tear ourselves apart when they can't be met.
Let me clarify what I mean by "burnt out" by attaining your PhD. You have said yourself that "the last year was the most stressful thing I have ever endured" and that is going to leave an aftershock. Your brain has been working to its limits for a sustained period and more than likely the only thing that kept you going over the final few weeks/months was the thought of the finishing line.
Well your expectation of reaching that finishing line was one of celebration and good times, but in reality you are that exhausted, that drained after pushing yourself that hard that you have nothing left in the tank. And your poor brain, that now only wants a rest, is now being berated/condemned rather than congratulated for getting you there.
Let me try to explain in another way. It's like you are running a marathon and after about 20 miles you are starting to fatigue - but the idea of the finishing line is pushing you on. So you dig deep and tap your "reserves" to get through those final miles. With a mile to go you are "shattered", but with iron willpower you force yourself over that finishing line, determined to get what you set out to achieve. Finally you cross the line and collapse in a heap exhausted. Now you achieved what you wanted, but do you want to get up and party? Or do you want to go to bed for a week?
You are burnt out mentally, you have nothing left to give, you have dug deep and tapped your resources and now you need to rest your brain - but instead, because you are seeing symptoms you don't like, experiencing things you did not EXPECT, you are still taxing your brain hard - worrying, doubting, searching for clues, questioning your sanity, etc. When the answer is simply to rest up and switch your brain off for a while, whilst it recovers from its marathon.
I have seen quite a few post graduates on here suffering from the same thing. Our brains, just like our bodies, can only endure so much and once they are taken beyond their limits then they begin to fail. When they do it does not mean they are now "broken" forever - but it does mean they need to rest in order to recover or to allow themselves to repair, recover, regain their composure, etc. but because it is the mind, the brain, people tend to think that they can keep on pushing, pushing, pushing and that there is something wrong with them if they can't.
People can accept this quite quickly when its their bodies they are talking about, but they can't seem to accept it the same when it is their minds - they can't seem to grasp that their minds can get "broken" - but they do. Now they don't break as such, but they do get out of balance, they get stuck producing chemicals they don't need any more, neural pathways have been bypassed, adrenal systems have been tapped into, etc in order to push you that bit further when you demanded it from yourself and now part of your brain still thinks its running that marathon, it has got stuck, but another part knows that it's over, so to speak, and so you will experience some weird crap until your brain/mind balances itself out again after this monumental effort it has put in for you - in order to get you what you wanted - but the aftermath of which you didn't expect.
You've got to understand your body, brain and mind and KNOW what you are doing to it to understand anxiety. Once you accept that brain now needs a rest, allow it have one and accept that it may shows signs of "malfunctioning" in the meantime, then you will be OK. If you keep resisting the signs of malfunctioning and therefore keep your brain tense, in action, on RED-ALERT (which incidentally is why you are feeling a low key uneasiness), etc. then unfortunately you will suffer from the anxiety of that.
Hope it helped