I play fender guitars strats & tele...Lately I have been getting a feedback...I have changed cords tried different settings still doing it..I even let someone use my amp and there was no feedback...Any ideas what it may be?
Tough one to answer without actually being there. There are a lot of variables.
Since it doesn't happen when someone else uses the amp, that leaves the guitars, the room, and the player.
Try to think what might be different since you started getting the feedback. Are you in a different room? Is the am placed differently in relation to where you are standing? Are you playing louder?
The only thing I can tell you is that you need to systematically eliminate variable until you find the cause.