Fitness

If it's sharp and in one place, and started at a certain moment, I would think of a trauma. If it came a day or two after and covers wider area, it's just your body not used to trainings and it won't reappear like that after a few trainings. I dropped gym and came back many times, and each time I felt "the punishment" for that :)
Edit: still, being in contact with a doctor is always nice. Don't go crazy about any negative feeling, but ignoring problems is always not great. Listen to yourself, but with a grain of doubt, because our bodies generally like to complain