Why am I getting stomach aches from coffee?

I’ve always drank a lot of coffee. But more recently I’ve had to give up my morning cup for tea because sometimes drinking coffee on an empty stomach would make me feel a little nauseous. I still usually drink a cup in the evenings.

Now, though, it seems like I can’t drink coffee at all without feeling nauseous, and also getting bad stomach aches.

Could I have developed an allergy? Or an intolerance?