What kind of bite could this be?

I noticed a small red dot on my foot earlier in the day. It did not itch or appear swollen. Within a few hours I was extremely head achy and nauseated. I tried to go to sleep and have been waking up every thirty or so minutes. Up until recently both legs have ached and I have had an upset stomach and the beginning stages of diarrhea. I noticed that the red dot has turned into a raised welt that looks like a mosquito bite and itches. It is no longer red. I do not know that all of these symptoms are related to the bite but it does give cause for concern. What could it be?