Why do I have a white spot on the back of my throat?

Do you mean the very back of your throat, or on your tonsils, which are one on each side of the back of your month where it becomes your throat. It might be that you have a tonsilith or tonsil stone - a white nasty smelling secretion that some people have a lot of in their tonsils. The tonsilith would make your throat sore in the area, but would eventually be dislodged and swallowed during eating or drinking. Some people poke their tonsils to dislodge the lump, BUT you risk giving yourself a nasty canker or sore spot if you do that. If it's a flat white spot, possibly with a red rim it's probably a mouth ulcer or canker. These may last up to two weeks and be very very painful, but they do go away by themselves eventually. There isn't any particularly guaranteed treatment to get rid of them sooner.