I had a massage once, and I was really sore afterwards. Why?

Massage increases circulation to body tissues similar to the way exercise does.  Lymphatic flow is also increased, which hydrates tissues and clears out cellular debris and normal metabolic waste products.  As these processes are increased, there may also be increased soreness, or even nausea, headache or other detox reactions as these things come to the surface.  It’s important to drink plenty of extra water after a massage to re-hydrate your tissues, most especially if you are recovering from an injury.