Hi, i am from south india. why some people have side effects like fever, vomiting etc. later on the day they took covid vaccine? body reacting to a foreign substance?

maybe: Any foreign material injected into the body may trigger a reaction to the basic material or the traces of preservatives that remain after production. As a biological material, like a banana, it has a functional time limit on full activity. Most vaccines will contain traces of a preservative that lengthens the time from production to use so it will still be effective at the point of use.

Immediate reactions to the vaccines are due to allergic reactions, and hypersensitivity responses, likely to the oils which surround the m-RNA. In some cases, preservatives are present and some reactions occur in that fashion. A immune reaction to a "foreign substance" would be delayed and not immediate.