I developed a backache yesterday afternoon (thankfully it had nothing to do with running), while doing some cleaning and rearrangement at home. This is the second time I have had this backache, the first was when I was trying to lift a box which was about 20kgs plus. On both occasions it was the technique which was faulty and not the work or weight. Yesterday, I kept my legs straight and bent from my waist while doing work and rearranging things, instead of bending my knees and doing it.
I usually try to avoid using painkillers and magic spray as they don't really cure much and give one false sense of relief. Its especially dangerous to use these when one is training, as they tend to hide pain/injury and one tends to push the body, when it actually needs rest. So I rested the whole of Saturday, with little bit of stretching and then waited till Sunday morning to decide on my ability to undertake the run (had it been a running related injury, I would have taken an additional day of rest). I also believe that one should not take any painkillers/sprays during a race or before a race as a precautionary measure (it somehow feels like cheating). The only time I have ever used sprays was during my first Ultra, not sure if they were very useful as they provide only temporary relief & when it wears off, the pain/cramp/injury becomes unmanageable.