This happened to me quite often when I was heavier, and I do still experience it occasionally. Heat seems to be the main culprit for me too.

I've found that drinking plenty of water and keeping active helps. Also, something else that works for me is sitting with my feet in a bowl of cold water.

I'm currently off my feet because I broke my ankle a few weeks ago, and have been experiencing puffy ankles due to my inactivity, plus it has been quite hot here in the UK. Along with some leg execises, which I can do while sitting and lying down, drinking more water and putting my good foot in a bowl of cold water has really helped.