What the hell just happened??

Is it possible to hit the wall after just 2 miles??? Because I think that's what I just did.

I set out to do a tempo run (tonight's Jack Rabbit workout that I'll be missing). I should have known something was off when I felt my legs burning as I jogged up to the park. Then there was the side-stitch that set in about a half-mile into the run. A slight headache at 1 mile. Then nausea and a feeling a total dead weight a 2 miles.

I stopped and walked home, somewhat disappointed, but mostly baffled. Some problem-solving:

Did I go out too fast? not really
Dehyration? perhaps, but I drank 2 large glasses of water this morning.
Am I sick? I don't think so.
The "heat"? It's only 68 degrees, but the sun was pretty strong, and it's possible that I'm just not used to it
Karma for playing hooky from work today? most likely

Ah the truth comes out...