This isn't a very normal movement to do and is usually associated with a anterior dislocation... so anterior instability was my leading thought - but if no problems with aprehension then I guess your ok with that. It's very hard to make any assumptions without seeing you. Given a positive speeds test biceps tendinitis is a possibility... The treatment your receiving isn't my first choice given there is little to no evidence supporting it. I'd say if it makes you feel worse then you should be stopping it and asking the physio what exercises you can do - assess for any muscular imbalance in the shoulder and also assess the effect of mobilisations of the thoracic spine. I'd suggest you look into some scapular stabilisation exercises and see the effect of those as well. Like I say hard to know without an assessment.