Yolinda,
My quick test for general joint laxity is to see if the elbows and knees lock in a straight position, or are hyper-extended. If they hyper-extend, probably most of your joints are on the lax side. In that case, I would expect you to have a higher propensity for joint injury and even dislocation. (Numerous sprained ankles and a dislocated shoulder would fit with that pattern, of course).
Because naturally loose joints are often more prone to injury, I think it’s especially important to help stabilize them by engaging the muscles around the joints while weight-bearing, and try not to lock them out completely. For example, in a forward standing bend, you would engage the muscle groups in the backs of the legs, and slightly back off the fully locked position of the knee joints. (Actually, I believe this practice of engaging the muscle groups being stretched and “micro-bending” the joints is appropriate for pretty much everyone).
Some teachers say the joints should not be allowed to hyper-extend at all, in a pose, so that the limbs are straight. While esthetically more pleasing, I’m not sure how important this is - you are using muscular action to replace the function of the ligaments in stopping the ROM of the joint, and it’s just not the same thing.
As for your back pain, I don’t think I’m ready to try more diagnosis with the incomplete information that a written forum allows. As I mentioned, a variety of things could be going on with your back, from mild to possibly serious. Someone would need to see what’s happening in various movements/positions, perhaps examine you by palpation and even do some imaging (such as x-ray) if the problem isn’t otherwise clear.
A yoga teacher may be able to help you get to the bottom of the issue, but in my experience, many otherwise excellent teachers may not be great at biomechanical diagnostics. Although I’m trained in acupuncture and tuina, a lot of what I know comes from my work as a neuromuscular therapist, from training with chiropractors, and yes, from a couple of excellent yoga teachers. Perhaps a physical therapist or chiropractor with an understanding of yoga would be a good start.