What Causes Elevated Hemoglobin A1c Level?

My recent Hgb A1C was 8.7! It was shocking since prior to this test, I always had a value of 6.5 and lower. Actually, my last value was 4.6 about 6 months ago. I have DM II, and multiple other dx: including RA and a severely affected immune system due to Gullian Barre. I recently completed a 30-20-10 dose of prednisone & am down to 2mg daily. I ll continue a tapering schedule until I m off of it. I ll continue methotrexate indefinitely for now @ 17.5mg a week. I hope I gave you enough info to answer. Thanks, Sue B