For the last couple of months my body temperature has been rising very easily and very fast after very little physical activity. what can this be?

By thermometer?: If your body temperature is fluctuating a lot by thermometer reading--then should see your Dr or an Endocrinologist, as my have thyroid issue or other problem. If feeling hot/flushed w/workout, that is body adapting to increased work to speed rate of cooling and is normal. Also, even w/oral thermometer, cool fluids can raise temp reading. Talk to your Dr for best answer for you.