Is my cat actually a boy?

We got this kitty from a friend, who found her as a stray, last summer. She took her to the vet. She told us it was a girl and the belly had been shaved with no sign of being spayed. There were definitely no visible testicles--even I would have noticed that. We took her to get her spayed last month--even though she never showed signs of heat. In the middle of the surgery, the vet called to say she couldn't find a uterus or ovaries, and assumed she had already been spayed. (We still had to pay for the surgery.) She healed up fine. But lately some weird behaviors have started (my kids actually say she has been doing this all along, at night when sleeping with them). Like kneading, biting and humping a blanket. I thought this looked like male behavior, and was kind of puzzled. Two vets she saw her and no one said anything about it being a male. Well, tonight she was sitting Buddha-style...and I saw it. A PENIS!!! OMG. Is my cat actually a boy? How could 2 veterinarians miss this??? My children cannot stop laughing whenver I now refer to Mittens as "him!"