Getting sick a few days after leaving for vacation ANYWHERE is very common, so common that doctors have a nickname for it, usually travelers' illness. Some of it has to do with the fatigue and stress associated with travel -- you know, the more fatigued and stressed you are, the more susceptible you are to getting sick.
But most of it is due to exposure to "foreign" germs. Foreign to you. Every community has its own breeds of bacteria and viruses, maybe only slightly different to germs in other parts of the country, but different enough so that people who live there have developed an immunity to them while people who are rarely exposed to them get sick more easily than they do at home.
You know the old saw about "Don't drink the water in Mexico," right? [I'm talking the nation, not the attraction in World Showcase!] Well, the people living in Mexico aren't sick all the time, because they have developed an immunity to whatever water-borne "bug" causes "Montezuma's Revenge."
So if you don't live in Central Florida, your immune system doesn't recognize the cold viruses and stomach bugs that live in Central Florida, and if your immune system doesn't know how to fight it, you get sick. But at home, it's likely that your immune system successfully fights off many of the cold and stomach bugs that live in your area, and you don't even know it.
Add into all of this the fact that the Orlando area is a HUGE vacation destination, so we as travelers not only have to deal with the native Central Florida germs, but also all the germs brought there from all over the country and other parts of the world by our fellow travelers.
It's MUCH easier to get sick after exposure to new "bugs," after exposure to germs that your immune system doesn't recognize and doesn't know how to fight.
My wife is a pediatrician. She spent her entire life, through medical school, in the city of Philadelphia. Being exposed to sick kids during medical school didn't increase her getting sick because she'd spent her whole life being exposed to Philadelphia's "bugs." And then we moved to the Cleveland area for her residency, 500 miles away. She picked up her first "bug" about a week or so after we moved, and I followed close behind. During our first year there, these two healthy people in their late 20's who almost never got sick were sick an awful lot. After our first full year in Cleveland, after we were exposed to all of the seasonal bugs, we went back to being our normal healthy selves, as our immune systems learned how to recognize and fight the bugs there. Then we moved to Atlantic City and went through that lovely cycle all over again ....