You describe cramps, not unlike menstrual cramps, after masturbation. Orgasm includes contraction of pelvic floor muscles, and it sounds like you're experiencing some spasms of those muscles. Radical hysterectomies often require tissue removal or dissection surrounding the uterus and ovaries. It's likely your spasms are caused within nerves and muscles that are still healing.
I suspect this will improve with continued healing, but using an anti-inflammatory medication like ibuprofen may help relieve the pain. If, three months or so after surgery, when most healing has taken place, the spasms and pain persist, a consultation with a pelvic floor physical therapist may be helpful. They can assess the muscles and nerves of the pelvic floor and often remedy persistent pain.
Continue that healing work! I'm hopeful the pain will resolve itself.