Pain, Girlfriend, and Sex?

Ok my girlfriend has a wierd tendency w/ sex. After I turn her on and things are underway, she starts inflicting pain on me. I don't mind her biting my lips, twisting my nipples, or even punching my gut (I have a decently hard midsection.) The bad things are she's started to try and squeeze my penis (when I was only a little hard) and she has tried to hit me in the balls (she didn't...I lied and pretended she did, which worked.) I'm not the least bit turned on by this I just deal with it. Why would she do things like this? Could I get her to stop? Could I deal with it? If I keep pretending will she get off just the same? Keep in mind she's Bipolar...and I'm scared she may also be a sadist and/or sociopath....Does this sound sadistic or sociopathic?