Peas or sweetcorn are especially good because the bag can be scrunched up to fit the contours of your back. Try applying an ice pack to the affected area. Do not apply ice directly to your skin, as it might cause a cold burn - put a wet cloth between the ice and your skin. If ice does not work, try applying gentle warmth, via a hot water bottle. But cover the bottle so it isn't too hot.
Some people find that alternating the heat and ice produces most relief.
But do try to get professional advice on applying heat and ice if you can. A hot bath or shower might also help. Take a painkiller Take painkillers as recommended on the packet - but never more than the recommended dose. Always read the instructions to make sure the tablets are suitable for you. Your local pharmacist can advise you.