Which food is the culprit?

I'm feeling ridiculously bloated and full right now, even though the last time I ate was about two hours ago. I made a mini pizza - one whole wheat pita, tbs ketchup, big handful of spinach, mozzarella cheese, and some garlic.

I don't think I've ever had problems with any of these foods before, but maybe I just didn't notice it? Do any of these tend to cause bloating?

Fiber in WW pita + fiber in spinach + dairy gas... the digestive system is not a 100% predictable machine, even if you've never had problems with the foods before. Plus maybe summer heat if it's hot where you are.

Dairy for sure! A lot of people don't know it but they are intolerant to the stuff and bloating is one of the signs!

Ketchup usually contains vinegar (acetic acid), which when combined with calcium gives off gas.

Did you check if any of the ingredients you used are expired?  If this isn't the case, I'd place my first bet on the cheese (dairy is just...gas inducing for many of us) and if that isn't it then the high fiber content or maybe just the bulk of the meal.  

It doesn't really matter what caused it, since this has only happened once.  I would start to worry, however, if you see a pattern of certain foods causing bloating or other problems.