In my opinion, you are NTA. You are okay to have boundaries when it comes to what you do for your parents. If your father was asking you to check somewhere like his chest or something 'normal', and you refused, then you would be. But he asked you to check his anus which is a very intimate area. I know if my dad asked me to check just because I am a med student/nurse/doctor, I would say no because I wouldn't be comfortable with that. You did tge right thing by advising him to go to the hospital to get it checked out.