You have negative AP because you had less than 10 when you stood up. Standing up costs 10 AP (unless you have Ankle Grab, a zombie skill that I'm saving up for) so if you have less than 10 to begin with you will go negative. Not a big deal, your AP will still recover at the normal rate, so once you've rested up a while you can move to a revive point. The nearest revive point is the cemetery a couple blocks south of you.
I would offer to revive you since I'm right there and have needles, but I won't have enough AP rested up for a few hours. Chances are someone else will have revived you by then, but if in a couple hours you still need a revive then post here and I'll get you.
ADDITION: note pack of wolves' message below: after being revived don't stand up until you've recharged enough AP to get to safety. Going negative when you're a zombie doesn't matter much, but if you go negative while a survivor you will probably just die again because you're stuck outside.
I'm up to 87 XP again so I'm contemplating my next skill. Right now I'm thinking about going for pistol training, since that should help me build up XP a lot faster and waste less ammo doing so. After that I'm thinking Construction, then maybe shopping or first aid. Sound like a good plan?
If you've been shooting and don't have pistol training then that sounds like a good plan. You want to build up skill in whatever your main attack is before branching into other skills.
Also, Diagnosis is much more useful than First Aid, get Diagnosis before First Aid.