My doctor prescribed omeprazole for stomach issue, and after taking it for 3 weeks i have a major rash. is it possible to take that long to react?

Yes: Rashes from medications can take several weeks to develop; some can even months or years after starting a medicine. This makes them very difficult to sort out at times, especially for patients on multiple medications. It is also possible that it something else, however, anything from a food to a viral infection can cause similar rashes, so a dermatologist or allergist can be very helpful here.