Wade, the same thing happened to me as well, with a slight twist.
Last night before I went to bed, my campfire kids were making skeletons. Between my neighbor visits and tapping skeletons in my own town I had enough to upgrade the campfire to the next level. I didn't feel like staying up for the kids to finish at the campfire so I went to bed.
This morning, the first thing I did was tap all the skeletons that generated overnight. Then I checked the campfire upgrade tab and it showed that I already had 26 zombies (same as you)! I hadn't even tapped the campfire or changed it to collect zombies yet. I have no idea where they could have come from. I thought I read somewhere that tapping nightmares in other towns didn't count towards your campfire upgrade. But, how did the campfire upgrade change from skeletons to zombies without me changing the campfire to produce them???
Oh well, it did give me a head start on the zombies. :D
Edit: I guess the zombies could have come from neighbor visits. I'm not picky anymore about whatever nightmare it is. I tap them all. Then if I haven't fallen asleep by the time the nightmares have run out, I keep tapping for donuts.