can the person holding my morgage borrow money against my home without me knowing it

Typicaally, no one can give a mortgage on property that he doesn't own or have some legal rights to. There are exceptions like leases (possessory right) which can be mortgaged, but your case doesn't sound like one of these exceptions. The mortgage might have predated your ownership and you took property subject to the mortgage. I'd need more information. Note: a mortgage is a lein but gives no ownership interest to the holder.