How Can Fatigue Along With Fever And Chills Be Treated?

74 yr old white male post aortic valve replacement and one bypass Sept 2017 doing great with exception of elevated heart rate upon mild exercise. Completed Cardiac rehab and feeling great. Diltiazem switched to Atenolol & Lisinopril on 2/1/18 with excellent results, Had been on both pre op so not new drug. Since evening of 2/1/18 temperature has been elevated daily, in the beginning 102 + with chills now 99 - 100.4 nightly. Feels lethargic as the day goes on and temp rises. Neg. for Influenza on 2/20, CMP - WNL, CBC- WNL (WBC -7.1), and urine neg. HX of Crohn s under control, takes Pentasa and Welchol. Any suggestions