Most minor illnesses are intensified by the mind. The placebo effect is what you experienced. The value of vitamin C and A has been drilled in our thoughts to an obscene amount. The mere suggestion of it's use as a cure for a general illness almost always produces a favorable result. (Mind over Matter) The fact is that the body is only able to process a certain amount of each vitamin. The excess is disposed of as bodily waste. Taking 1000mg tablets of vitamin C has the same effect of a 300mg daily dose. (Again, mind over matter)