Mr. A do you have health insurance?
If so, get to a sports medicine doctor, and tell them you are interested in seeing a physical therapist. PTs fix physical imbalanced and posture problems like you are describing for a living, and you sound like a classic case of someone who needs to see a PT.
See an MD of some kind first, and they will not just tell you to "keep off it, and call if something happens", more likely they will refer you to a physical therapist who will prescribe specific exercises and stretches that will fix a lot of your problems in a matter of weeks (prob a month or less if you are not seriously injured already), and protect you from serious injuries later on. If the MD doesn't take your problem seriously enough, TELL THEM that you really want to see a PT, they will almost certainly refer you. Yes, you can do exercises and the like yourself, but at the very least the PT can give you a good idea of what it is you need to do- without their expertise you will be blindly stabbing at the problem in the dark. If nothing else, just visit them one time and get a general idea of what is going on and how to improve it.
Trust me, get this stuff figured out now. If you ignore the minor aches and pains and various imbalances you'll be OK for a while, but when you get to your 30s (I am assuming you are a bit younger than me) this translates into acute injuries requiring surgery. If you are already in your 30s, it's even more urgent. PT people know their stuff, and their prescriptions work. Their work is often complementary with things like yoga or other kinds of exercise, and you can consult them about exercise and physical activity beyond what they prescribe you.
It might cost a few hundred dollars at least (US prices, with health insurance), but trust me, when you've suffered an acute injury and are afraid you will never be 100% back to normal again, that money won't seem so important.
Sorry if I am lecturing you, but I've been through some back issues over the past couple of years, so I have some idea of what I am talking about. It seems that people generally don't see PTs unless they have already had some kind of catastrophic injury, which is a pity, because if people were to visit them earlier they'd save themselves a lot of grief.