My Dad also has diffuse large b cell lymphoma. He had his first CHOP-R treatment 4/18/08. He too done really well the first two or three days, he actually said he felt better than he had for long time - he has arthritis, and a lot of the medication included with the CHOP is anti-imflamitory, plus the prednisone. The doctor said that is why he was feeling so good. About the 4th day after tho, he wasn't doing so good. It scared me, so I called the lymphoma org and the cancer society, and they both said this was fairly normal - other than the headaches he was having, but then again, he was constipated, so the headaches could have been from that. They said to make sure he got plenty of liquids. They suggested ensure too. Also, pedialyte if you can get him to take it since it has the added electrolytes.
Good luck, and God Bless!