PredniSONE Stomach Ache, Bad Taste in My Mouth, Crohn S Disease, Imuran, Mg Dose

I have crohn's disease. The first time I took prednisone was 4 years ago when I was first diagnosed. I started with a 40 mg dose and then gradually decreased it for 2 months. The only side effects I had then were a round face and acne which went away when I stopped the medicine.
Recently I had a serious flare, and the doctor put me on prednisone again. I will take 40 mg for 2 weeks and then decrease it by 5 mg weekly. This time I'm having very terrible heartburn and stomach ache. I also have a bad taste in my mouth and am always dazed and cannot concentarte. I started taking also Imuran , which my doctor says will help me and allow me to stop the prednisone in 2 months. I hate this medicine!