Assuming your bike fits you properly and saddle height/fore and aft has been adjusted correctly plus handlebar is set right, I'd say give your butt a chance to get used to riding. Your butt, at this early stage, is wondering what the hell is going on.
Beginning riders usually have pain for a few weeks. If pain persists or becomes severe then take action.
As joeprim suggested, stand and pedal frequently; that can make a big difference.