I Am A 73 Yrs Male, Treated 2.5 Years Ago

I am a 73 yrs male, treated 2.5 years ago with proton beam therapy for prostate cancer, 5'-6" tall, 210 lbs, set for hip replacement on May 4th, with arthritis pain severe enough in hip to require wheelchair on way back to hotel tonight. I just noticed severe swelling in ankles and feet, never noticed before.

What might be causing this foot & ankle swelling ?

CBJ