Leftside is very possibly right about the guy being a moron. On the other hand, he may have decided that the best thing a soft-throwing lefty could to is become a side-winder. While it's generally not a good idea, IMO, to change a pitcher's arm slot, it's also true in general that side-arm is less stressful on the arm.
In your case I'd talk to the guy away from the field - ask him why he thinks you should change your slot, talk to him about the work you've done to increase velocity, and make sure he knows about the pain you've been having.