I'm having severe backache at the left side accurately i tried putting a hot water sac on my back it helped but it's been a week and it won't go away?

Strain: Can be strain, muscle spasm, kidney infection or stone, herniated disk, etc. See physician.

Need physical exam: As you are 17, and the fact this could be so many different things, recommend seeing a physician and getting assessed. It could be anything from a muscle spasm to kidney or infectious issues. Be careful with heating modalities over a long period of time as it can cause skin damage and something called erythema ab igne. Hope it gets better soon!