I got a red rash in 3 patches on my chest and stomach after a colonoscopy. What could have caused it? Should I be worried?

Unlikely. If it was related to the colonoscopy, it is because of a drug reaction but allergies to the typical sedation medications used is uncommon. I would recommend following up with your primary care physician to get these areas evaluated if still present. They can make a more accurate diagnosis by getting your history and examining you.
Did they. Did they use any monitors on you? You can be allergic to the adhesive in the monitor pads.