I would have thought that the treating physiotherapist would have know that they are placing the shoulder in an unstable position. Normally we would do this albeit carefully and holding close the head of the humerus to get a feel of the subluxation. It is a little hard to tell from what you have describe if the treating physio was performing an instability test or if they were challenging the joint perhaps a little to early. Is isn't something I would do so early post dislocation myself. All that said it is likely that it would have popped out again doing something as simple as scratching the back of your head etc. The indication that is did happen is one for an orthopaedic consult and possible surgical tightening.

Do let us know how you get on.