I've been sick for the past two days. Not like anything else I've ever had.
But, I'm on the mend now. So, the question is: What was it?

Symptoms:
Slightly elevated heart rate on Thursday. Like I'd had too much caffeine, but I hadn't. I actually drank less coffee than usual, because I already felt nervous, because of the elevated heart rate. (Not the other way around.)
My resting heartrate is usually 60bpm.
I measured 68 on Thursday. And I measured it because I could FEEL it.
No appetite for dinner Thursday night.

Saturday, I woke up with a more elevated heartrate. 86bpm at 7am.
That jumped to 127 after climbing a flight of stairs, like I'd just run a mile.
And I had a headache. Not "splitting", but bad enough to be distracting.
Shaking my head hard was enough to aggravate it.
I never get headaches.
And I was running a low grade fever. I measured my temperature at 99.4F.
Plus, I was just feeling crappy in general. So, I called off of work for the day.

I don't think my temperature ever broke 100. And heartrate was around 88 all day.
But, my temperature was back to normal (98.5) by 4pm.
And, my resting heartrate was down to 75bpm by then.
By midnight, it was down to 63bpm, and starting to feel normal again.

No respiratory tract involvement. No runny nose or coughing.
Minimal GI tract involvement. Loss of appetite mainly. Occasional abdominal pain, once or twice, early in the day.

So, like I say, the question is: what was that?
I had my flu shot this year.
Is that what it's like to have the flu when you're already immunized?
The heartrate thing...that really bugs me. If I can avoid that in the future, I'd like to.