I have poison oak and poison ivy, im taking prescribed prednisone for it and now im breaking out in cold sores?

Stop prednisone: Prednisone suppresses your immune system which can reactive HSV-1 (the cold sore virus) which normally remains inactive in your skin for life. I would recommend stopping Prednisone and switching to a topical steroid to treat the outbreak. There are topical and systemic therapies that can be given to suppress future cold sore outbreaks and I would discuss these with your doctor.

Cold sore flare up: Steroids are a double edged sword. If your rash was really that bad probably you needed the steroids, but now the immunosuppressive effect has allowed a different problem to arise. The cold sore virus, HSV-1, is always waiting to come out when the body is weak--through stress, other sickness (fever blisters) or medications. Hope u can wrap up the steroids soon.