Dear User, Thank You for choosing Health Care Magic, I do understand Your concern. There are many people that experience back pain as part of flu symptoms, but You explained that you had it before the flu. In your case what happened is that you used to suffer from back pain (chronic or acute, I will explain later), and the flu excarcebated it, so made it worse. When the flu will go away (consider Ibuprofen, paracetamol, home remedies for treatment), the back pain will release or will get at the phase that was before you had the flu. If you had this back pain in one side for the first time of your life only three weeks ago, please remember if you injured your hips, or if you are doing exercise that stress your back, or if you are staying sitted in one place for too long. If none of the causes mentioned above is not true, than I would suggest a urine analysis to check the condition of your kidneys. Mean while, I would recommend some topical antiinflammatory cream to relieve the back pain and also any heating pad, or specific belts which you can find in drugstores. I did this explanation to clarify you that your back pain did not cause the flu and there is any other reason beyond this two conditions. The back pain, in your case was strengthed by the flu, and will go on after if you are not accidentelly hurted or stressing your back. To make it go away follow my instructions mentioned above. I wish You all the best, Dr. Eni.