Well, in my experience, there are three kinds of pops that happen during workouts. Good pops, bad pops and really bad pops. With a good pop, something is moving from a bad place to a good place (my right knee in October 1999). With a bad pop, you've pulled something, but it will heal in the ordinary course (lots of stuff). A bad pop is a season ender or just a super long recovery (glute pull and resulting sciatica 2004).
This morning was the 6:30 AM plyo class. Did I mention I ran a race that ended at 11:00 pm last night??? Anyway, I made it to class, and actually felt pretty good. Warming up, my legs were loose. I didn't stretch as much as I should have, but I'm not a big pre run stretcher. Oops!! About 10 minutes in, during a segment of walking lunges, i felt a "pop" in my left hip flexor. . . This has been a problem spot for months, as I have been trying to even out my stride. I stopped, walked it off, and found that I could do most of the exercises, except for lunging. That hurt like heck. As the day has gone on, it has settled down. I won't know for a few days which kind of pop it was, though. I think I won't attempt my planned 18 miler, and will stick to pool and bike for a couple of days . . .