Post-nerve-sparing prostatectomy, I took 10 mg Trimix but having a painful erection. Why?

Hi doctor,

I am 63-year-old. Five months post nerve spearing robotic radical prostatectomy, and I was prescribed 5 mg Cialis daily to maintain blood flow. After meeting with my penis rehabilitation therapist, I was prescribed Trimix. The initial dose was 20 mg. After 15 minutes, I got an erection, but 30 minutes later, the erection became painful. Two hours later, I started using an ice pack and took four Sudafed. I call the nurse and informed her of what happened. She recommended reducing to 10 mg. four days later. I tried the 10 mg injection. 10 mg might not have been enough because it produced a weak erection. But it was still a painful erection. It lasted for one and a half hours. I talked to the nurse again, and she recommended 15 mg. I stopped taking the Cialis three days before the injection. The 15 mg produced a stronger erection but still painful. After two and a half hours, I took Sudafed and used an ice pack. The erection continued for over three painful hours. No matter the dose, it seems that the Trimix injection produces an immediate painful erection that lasts for hours.

Before my surgery, I had no ED issues or any type of painful erection. I cannot seem to find any information that describes what I am experiencing. Everything I find deals with priapism, but the pain occurs after a number of hours. My pain comes minutes after I achieve an erection with the Trimix injection. Is this normal? Does the Trimix need to be changed? Kindly give your opinion.