When one works out with vigor, you tend to be a little sore the next day or two days after. Most cases you can think back to which exercise caused the sore muscles. But sometimes you move and a muscle is sore and you have no idea why. You start thinking what the heck did I do for this muscle to be sore. Was it something in kick boxing. What combinations did I throw? Was in grappling? What moves were we working on? Was it during strength training that I pushed this muscle to the limits and beyond? Sometime you just can't recall. I am not sure if is because I am old or what.
But sore muscles are good. Knowing you worked out hard is a good feeling. Progressing to your goal of a healthier fit you. Waking up the next day in serious discomfort and pain is a bad thing. That just means you over did it. Now you need to take time off from training to heal. Slowing your progression. Very frustrating.
Work out hard but don't do damage. It can be a very fine line.