I am not a doctor. Nor physical therapist. Nor any kind of licensed health worker. I am a certified yoga instructor. I am a science-curious, anatomy-fascinated, yoga practitioner who studied so that she could share information about yoga with other people in a safe, productive, interesting and functional way. I am learning about how the body works (or doesn't work) through my own practice, and self-study, and hope to inspire other people to do the same.
That being said, I MAY have had an experience or studied about an experience that MAY be able to help you improve your practice AND prevent injury during it. I MAY, however, not be able to.
Some yoga instructors in fact ARE PTs or MDs or PhDs in the health profession and may be able to not only diagnose a problem but also offer a route to wellness.
So, if you have a condition/problem/ache/pain that has been bugging you for more than, oh, say, two weeks and hurts MORE when you practice yoga (or when you are still), PLEASE go see a qualified medical professional to diagnose the problem and hopefully send you down the path of healing that may include a yoga practice.
Some modalities of self care and healing may be more obvious.
1 - If it hurts when you engage the muscle, it's possible you have injured the muscle, the tendon, or fascia. Stretching may NOT be the smartest option. You just may need to REST it. Take an Epsom salt bath, drink a lot of water, do the anti inflammatory meds thing if you are so inclined. Exercise in a way that doesn't exacerbate the discomfort. Seems obvious, but I can identify two stories just this week where someone had pain when using a muscle, told me it had gone on for years, yet they continue to overwork the area of discomfort instead of letting it heal/rest.
2 - If you have a lump, a bump, a bruise that won't go away, or a sharp pain when you move, PLEASE see a doctor. Growths that cause pain, things that don't heal, and sharp pains MAY be indicative of something that may not be fixable with a lunge pose or just a deep breath. They may be nothing, but seriously, if your child or best friend winced in pain when you touched something on their body, wouldn't you recommend they go to see a specialist versus try to just "asana it out?" It may be a tumor, it may not be. But you won't know unless you check it out.
3 - If you take a pose and you feel like it's your bones that just won't let you move more deeply in a pose, it may very well be that you have an anatomical structure that disallows that variation of a pose. Few folks have hip joints that allow opening the legs to 180 degrees, and as few have shoulders that rotate fully with arms parallel. Sacral joints are ones of stability, not flexibility (quote from Judith Lassiter). Destabilizing a joint without proper support (and instruction in a practice like Yin yoga) can, yes, lead to injury, so, be okay with where your body is (and is not).
4 - If you get really dizzy when standing up quickly or moving from standing to bending forward or vice versa, you may have a blood pressure issue (If you get nauseated this could mean something more dire - see a doctor asap). Make sure when you stand up from a forward fold or squat that you move on the inhales, to help counteract drops in blood pressure. If you typically have low blood pressure, don't expect your body to "fix" this - enjoy your sought-after systolic/diastolic status and stand up SLOWLY to minimize the chance you'll end up back on the ground without warning.
5 - Drink a lot of water, even during your practice. Yes, some practices of yoga dictate when you can drink water and when to take breaks, so if you like that dogmatic style then fine. But even if you are participating in those practices, if you feel faint, dizzy, weak, or sick, then DRINK, for heaven's sake.
6 - Let any injury heal, and know that it may change your practice drastically once you do, or it may not change it at all. But you have to be open to observing how that injury affects even the small movements, and be willing to start slow and be patient. The bigger the injury, the more acceptance you may have to do. Hamstring tears take months (or longer) just to heal, and overstretching or overworking them too soon will just re-injure.
7 - Contrary to popular tee shirts, pain is NOT weakness leaving the body. Pain is the sign something is amiss. It may be small or large, but you need to go within to figure out if it's physical discomfort, emotional discomfort, small ache, or real pain. Be honest with yourself, try not to react but rather observe, whatever the scenario, Be open to solutions that your intuition offers and try not to second guess yourself. Ask for a professional opinion, Don't judge yourself for taking care of yourself. And don't suffer. PLEASE.