Grapes can make you bloated because grapes are high in a natural sugar called Fructose.
That Fructose which is a natural sugar can cause gas buildup in your stomach which causes bloating and some people have a harder time digesting that fructose sugar than other people.
So some people can experience more bloating from eating less grapes while others may only experience bloating when eating too many grapes.
It could be that you're eating too many grapes which is making you bloated and possibly have gas but if you're only eating a little bit of the grapes and getting bloated you may be having a harder time digesting that fructose.
Grapes also contain a lot of tannins which can upset some peoples stomachs more than others.
However grapes are healthy but they an also lead to stomach pain, diarrhea and yes the bloating.