Is it really necessary to do confinement or is it just a Chinese practice?

I wouldn't say that it is a must but it would be definitely good for you and your body in the long run. Otherwise, the tradition wouldn't stay for generations after generations. For me personally, I was like Jamie. I only did the ones that I felt were making sense and omitting the ones I felt that wasn't nessary (no washing of hair, ekk!). I wore shorts because my house had no aircon, I showered twice a day. But I did have confinement food + post natal massage & tummy wrap. So far, I feel pretty much fine even after 7 years.