Yes, high activity can encourage an infection if for no other reason than the constant stretching of the incision site. That will allow for bacteria to enter much more easily and will also cause her to lick the site eventually due to the irritation of the pull that is caused by her running around like a Yorkie! lol
Yeah, so I'd either keep her on your lap as much as possible, in the crate at your feet, have on a leash in the house, whatever you need to do to keep her w/ you for the next three days easy. It's no fun when/if an infection does sit in. And besides, if there is a lot of trauma to that area, internally her tissue will be affected and remember that as I type she's developing scar tissue, well she'll develope more of it if there's constant pulling and tearing just by her extending like she does naturally if jumping or running up stairs.
As far as her chewing at her butt periodically she should've only received one shot back there if you gave her a normal series. If so, check to see if she has a small pea sized lump under her skin at the injection site. If so, I personally put a warm damp cloth on it for about 5-10 minutes as often throughout the day as I can manage and it seems to go away by the end of the next day completely and my dogs seem to appreciate it! After the lump has formed, I don't massage it either...it only seems to irritate them further, the warm compress seems to do the trick!
Hope this helps!