Broccoli can upset your stomach especially if you have an allergy to broccoli.
Some people are allergic to broccoli and other green vegetables and they cannot digest the broccoli properly.
Other people may just have digestive problems and naturally just cannot digest the broccoli.
Cruciferous vegetables, like broccoli as well as cabbage, have the same sugars that make beans gassy which can make the broccoli as well as cabbage hard to digest.
Raw broccoli tends to be harder for some people to digest than cooked broccoli.
If you continue getting sick from broccoli then it would be best to let your doctor know as you could have an allergy to broccoli.