Welcome @pawprintpeg, This is a question that you really need to discuss with your rheumatologist or doctor. I regularly do intermittent fasting but my PMR is in remission and I'm no longer on prednisone. Here's a reference that might provide a little information on the question topic.
-- Fasting with adrenal insufficiency: Practical guidance for healthcare professionals managing patients on steroids during Ramadan: https://pubmed.ncbi.nlm.nih.gov/32419166/

You mentioned that you have only been on prednisone for a couple of weeks. Have you been diagnosed with PMR? Is the prednisone helping with the PMR pain?