It is quite natural although your backache seems early. I have my backache starting from 8th week. you can try to ask someone to help you rub your back for you. you can also try to buy those pillow for pregnant woman (i got mine in mother world) to help support your back when you sit or sleep.
Try to bath with warm water and it will start loosen tightness of the backache. and also you can go for a swim. not exactly swim, just soak in the water. will feel more relax.
i think taking panadol is not very good for your baby as well. so is best not to take too much if possible. =)