I went to another doctor yesterday afternoon and I finally found out out what I have - shingles.
So it isn´t just an allergic reaction to the painkillers, it´s a virus, which explains all of my other symptons such as sight fevers and head aches.
Now I´m on antibiotics and will rest when we get to Ometepe, which is our next destination. We´re getting a taxi to the dock and then getting a small boat over there. Hopefully it will be relaxing enough for me to start feeling happier - I just want to get better and get on with my trip!!
I took the anti-biotics and used the new cream for the first time last night and as a result, I had a better sleep than I´ve had in quite a while. I hope the improvement continues. :) I´ll update again after we arrive on the island; we´re leaving this morning.