So glad to hear you're feeling better! Keep in mind that just a little less than four weeks ago, your insides were mixed up with a beater. Your stomach was pulled up and around your esophagus and stitched there.
A slice of pizza is a lot of food. Your stomach is complaining! Next time, try a little pizza, and later, a little more, etc. Also, if it had spicy toppings, it could be irritating to your still healing stomach. It'll take a while before it gets used to its new configuration. It will...just be patient!
So glad you're enjoying real food! Just take it easy, and you'll be just fine.