Just finished a medrol (methylprednisolone) pack. Now I have back, flank and stomach pain with nausea, dizziness and low grade fever. Could this be a side effect?

Doubt it. Anything is possible in the universe but I wouldn't put my money on all of that being SIDE EFFECTS of the MEDROL (methylprednisolone). I'd get a urine analysis, CBC w/diff, some orthostatic blood pressures, and make sure I were drinking doubling my fluid intake with the symptoms you've got. Oh, almost forgot, make an appointment with the doctor who gave you the Dosepak to check you over for an infection.