Anyone know why I have constant back pain?

I know I shouldn't have, but now I've looked into it I've scared myself into asking this question haha - so, I've always been tall, like, really tall. I'm 18 in about 6 months and I'm nearly 6 foot 6. However, it seems like it's been forever, but I'd wager since about 13/14, I've had a numbing pain in my lower back, and tenseness and aching in my upper back and shoulders. I can still do everyday activities, I can lift heavy stuff, and can just generally use my back, but it's still a pain to have. In some cases, this can be quite literally as a sharp pain, like a lighting bolt, can jolt through me when I bend down. It's only rarely, but it can happen. It feels good for about 10 seconds if I 'crack' my back while stretching it, but then then numbness and aching return. Lastly, I don't know if this is related, but around half the time, if I either sit or just bend my right leg, I get an atrocious pain behind my knee, like, a spasm, but it can be so painful as to cause tears, so I don't know. Anyway, thanks everyone!