I am seeing a podiatrist for a split tear in posterior tibial tendon, wearing a boot and avoiding weightbearing. why is pain worse after rest. although i am wearing a boot, even if i take it off, ambulation is not as painful as after rest?

It’s a very interesting diagnosis and I assume all the appropriate MRI and ultrasounds were performed. During rest and Nonweightbearing the soft tissue will shorten and tighten. Upon weight-bearing the soft issue and tendon will stretch. I might keep immobilized in a cast if tendon is torn to allow healing at the proper length.

Suggest you see a fellowship trained MD specialist in foot/ankle surgery, i.e., an orthopaedic surgeon.