Loud feedback noise problem, (not using anything acoustic)

I'm using mainstage 3 as an extension of my Nord Stage keyboard, so I only use some sounds that my keyboard doesn't allow me because it can only play a certain amount of sounds at the same time. The thing is that I'be been using headphones and mainstage sounds stops and they are replaced by a typical feedback loud noise that almost makes me deaf. And this weekend I wasn playing live on a big stage and suddenly the same happened, everybody were checking the mics to find the problem but it was just me. So I'm not even using a mic on mainstage or anything. It looks like a bug or maybe something that I don't know. Can anybody help me with that??

Thankyou.