How to go from pt time regular, to PRN ??

I have been working at a hospital part time, regular schedule for just over 3 months now. I am counting the 6 week, full time orientation period. So I'm just past my 90-day probationary period. In that short time, I had never called off or been late to work. Unfortunately, I have come into some health problems. I still don't know exactly what's caused the trouble, but I have been so sick and tired that I've had to call off a few times in the past two weeks. Currently seeking treatment/care, but we don't yet know what the underlying problem is, and so have no idea how long it will take to be managed well enough for me to work! Is it at all appropriate to ask my manager or HR to go PRN so I can at least hang on to my job??