That is weird, I’d have to say. You have the right to consider your offer until 4/15, unless other specific dates were specified from the school you were admitted. If it were me, I would politely reply to the professor that you are still waiting for some other results from other schools, and would appreciate if you could get a bit more time to think about your decision.
But if you are quite sure that you want to work with that professor and if that’s your best choice, I say accept the offer. No good in having an arguement with your future advisor.