Is it normal to be attracted to an ex-gf's mom when she hits on you?

ok, this is a weird one.
i went over to my ex-girlfriend beacuse she wanted to talk to me about something,so, we started talking in her room, and she went to the bathroom. I look at her doorway, and i see this leg slowly making it's way in. and i thought it was my gf, so i thought"wth, what is she doing??" then i see, it's her mother. so i start to think "shit, i'm getting a boner from this." she walks over to me and she just sits down on my lap, and starts moving around.then a little later my ex is out of the bathroom and was heading back , she looked at her door and she said "i'll be back in a sec!" and runs downstairs. her mom walks to the door and puts her pointer finger to her lips. she walks out and i still have a boner IIN? (help)