Transistor series regulator heating
11-13V to 5.25V @ 50mA linearly regulated is 0.2875W to 0.3875W dissipated.

If ambient is 25C, then a temperature of 49C is a rise of 24C above ambient. 

That works out to a junction to ambient thermal resistance of 126C/W. The BD139 has a listed thermal resistance of 100C/W without heatsinking. So that seems pretty close.