Stuff a sock in your mouth, learn to tone it down, or stop having sex when she's in the house.
I can speak from first hand experience that it is INCREDIBLY awkward to have to listen to your mom screwing some guy... Even if it is the live in boyfriend.
At 11, I REALLY hope she already knows about sex. If not, you need to get on that. I highly doubt that she actually thought he was hurting you- chances are she was fed up with the noise and that was the least awkward excuse she could come up with for interrupting you.
Seriously though... If she is mistaking your sex sounds as *screams* and cries for help, you have some serious issues going on... If you are old enough to have an 11 year old daughter, then I would certainly think you are old enough to practice discretion and understand that SCREAMING out during sex is inappropriate with a child in the house.
Smh. As for what to say, you apologize for disturbing her, and tell her that you were having sex and got too excited. Then don't ever put her in that situation again.
ETA: I don't care what the relationship is, you practice discretion during sex when there are children in the house. My husband and I are able to enjoy a healthy sexual relationship WITHOUT disturbing others in the house. I certainly don't have any judgements about sex with live-in boyfriends... I was doing the same thing when my husband was my boyfriend, and my brothers (ages 19 and 3) were staying at my house, and were able to be discreet. Married, We either keep the noise down, or go somewhere the noise won't carry to our child... I'm sure she does hear the stray moan or two, but certainly nothing to the point that it would cause her to come to my room in tears fearing for my safety. When she's out of the house- game on and we can get as rambunctious as we want. But when she's home we make a point of trying not to traumatize her.