Can my employer legally make me work for more than one of his businesses, if I was only hired for one?

I am a college student, working PT to FT (depending on the week) for a local inn, in the North East. It occurred to me today that my employer also has me doing several jobs for his real estate business, on top of my work in the hotel. This has never occurred to me before, but I’ve recently been presented with a few other questionable acts, leading me to believe that perhaps, I’m being used. Possibly for, not only two, but several separate companies, without being hired for or paid to do so.
Is this legal?