I was sick &moved little for 20 months,now my shoulder/hip/knee joints pop constantly &feel like theres little support, tendons seem weak,why is this?

Deconditioned?: I'm not sure why you hadn't moved for several months. I'm going to assume you were ill. Sometimes being sick for a long period of time and being inactive weakens the muscles. This is generally referred to as deconditioning Muscles need constant work to keep it strong. I would suggest a strengthening program directed by your doctors and physical therapists. good luck!