What Causes Painful Unwanted Erections?

I m a sickle cell anemic patient. I ve been living very fine cos I ve been receiving utmost care coupled with good nutrition and I drink water a lot. I was researching on the effects of too much bilirubin in the body when I came across the word priapism . I was shocked as I used to think those unwanted erections I had were normal. But after conducting more research on priapism, it doesn t completely fit my situation. It s slightly painful, never lasts for up to an hour, I doubt even half an hour. It actually added to my masturbation habit that I m trying to get rid off now as that seemed to be the only solution to make it die down, even after ejaculation it still stays slightly erect for a few minutes. Whenever I sleep off or get absorbed into another activity, I wouldn t even know when it died down. I just want to know if I have anything to worry about. I m just 18 years of age.