If I just slipped on the ice, landing on my backside
 

Im an 18 year old, 140 pound Caucasian male with asthma, allergies and post concussion syndrome
Kinda slipped on the side and landed on my butt/side of my legs/hips. Didn't hit my head but I felt the shock in my upper neck and its a little painful and tense now. This happened about 20 minutes ago

How worried about this should I be?

First dont worry, second worry has never solved any problems....

Ice your neck if its hurting, watch some tv, get your mind off it...

I hope you feel better soon, and try to be a bit more careful!.....