Although I would encourage you to continue to look into it, I wanted to mention that 1 year ago I had an allergic reaction to a bee sting and while I was being filled up with benadryl and medicines I became very faint and nauseous, and started tremoring so violently, with trembling in the muscles of my limbs you could actually see, that the whole hospital bed was shaking. And I couldn't stop it. By and by it calmed down and then a few days later after I went home I became extremely nauseous again, clammy, feeling cold, very very faint and tremors. I thought I was dying, actually. A friend was calling my mom when it hit and mom told her I was begging to be taken to the hospital and she handed me the phone and said my friend said "You are having a panic attack (she listed all my symptoms without even having to ask what they were) If you go to the hospital, they'll run tests on you just to tell you that you are perfectly fine. Because you are." And as soon as she told me that, the symptoms started disappearing!
Since then I've still dealt with them, when I'm already worried (which if I'm honest is most of the time) and I start worrying about my health, (once when I thought I had food poisoning, that one ended with me fainting, and then when I had my post concussion syndrome starting up, which is why I'm here, and I didn't know what it was- it was very very frightening) the symptoms would return, uncontrollable trembling, feeling locked up, nausea, cold, perhaps sweating. Sometimes I know what's happening and I can talk myself down from it, sometimes when my fears are just getting the best of me I can't. I'm going to be looking into getting counseling to help me, maybe there's a medication that's very good at helping control panic attacks. I don't want to live at the mercy of this.
I've learned that they are NOT the same as anxiety attacks, I've had those before too. Anxiety attacks are uncontrollable emotions wreaking havoc with how well you can function, while panic attacks can happen when you feel fine but deep down you're afraid and your body begins to react to that as though you're absolutely, utterly, completely petrified.
NOW, I do not know for sure if this is what you have been dealing with. I am not a doctor. BUT since you've had tests done that show nothing wrong, the doctors say "stress", and the symptoms are so very similar to mine, I wanted to mention it.
Remember to relax and be kind with yourself from time to time, we all need it. Sometimes when I'm relaxed enough to enjoy myself, I just start crying and crying. But that's still good- I've got a lot bottled up inside. I used to think I was one of the strongest people I knew, almost blessed with emotional and spiritual strength. But then when I started dealing with things that really incapacitated me, I found out that we're never as strong as we think we are. All we have to do is be JUST strong enough. And that's much easier.
So, keep getting those tests if that will make you more at ease. But don't discount the possibility that you just need to let go of some "worry" habits (like me!) and be good to yourself. I have my concussion too and I'm just taking it one day at a time and changing my perspective.