adverse side affects of rapid weight loss.

January 1st. I started walking for health reasons the first day 1 mile.which I've increased to 3 or more miles every 2 days. Feburary I started a diet very low in fat 10-20 grams a day instead of 300 grams. my weight loss is great 25 lbs. in 5 weeks! I have 100 lbs. to go so I don't want to stop or slow down but I have side affects that I could live with out. Urinating burns and AZO standered is not helping. could this pain be a result of burning fat too fast? hanging skin is another side affect that I can live with but if there is somethig that I can do besides wait or plastic surgy?