For reference, that is the stretch I was doing, except I bent forward, grabbed my toes, and helped pull my legs apart a little further, before pulling my body to stretch farther. I’ve been stretching like that for the past few weeks, but a couple of weeks ago it started giving me trouble. I decided that’s because it wasn’t being stretched enough, and went a little harder with my stretch last Friday. I realized that was a dumb thing, as it hurt pretty much immediately, and was still a little painful for the rest of the day.
Did I screw up badly? Should I just stretch more, but gently (read: properly), or stop being active for a while?
Well as a general rule, you should focus on stretching for a purpose–excessive flexibility can lead to joint laxity, which can lead to injury. While some areas can benefit from general mobility, you should be stretching for a purpose.
Here are two articles that you should definitely look at: