Maybe cancer, but probably not... Help?

This sounds like a blood pressure problem. You probably are having low blood pressure. If you recently had knee surgery (I did too) and you are not walking a lot, your blood pressure can drop a little and you can feel these symptoms when you stand up. It may also be the result of any pain medications you may be taking. Those can cause all sorts of side effects including dizziness, headache, vertigo, ect...Scoliosis may be related to your back pain, but that could also be due to the position you are sitting in. Since you are young, and going through puberty, you could also be anemic. That would cause the cramping, weakness, and low blood pressure. A lot of teens are anemic and need extra iron. Basically, all of what you are experiencing sucks, but is more likely explained by other basic medical things than cancer. Cancer in your age is very rare. Also, the most common symptom for cancer, especially in children, is being tired. Your body is recovering from surgery, be patient and don't worry. If your symptoms don't go away, tell your parents and ask them to take you to the doctor.