If your butt gets the least bit sore, or uncomfortable, or feels odd, get a new saddle IMMEDIATELY. Whenever you get a new bike, that's always something that might have to be swapped out. Don't put up with butt pain. Believe me on this. Even if you have a cheap bike, a (somewhat more) expensive saddle is worth it. You might be able to keep that new saddle and put it on another bike that you purchase later.
I disagree. If you haven't been riding regularly, it is not unusual to have a few days of adjustment before you get used to sitting in a saddle. If it persists though, definitely start looking for a new saddle, or at least see if you can adjust your existing saddle better (if you're sliding forward or back, change the tilt, if you're riding the nose or tail, slide it fore/aft, etc)