Why did my mom get so pissed off from the birthday my girl best friend gave me? I'm so confused?

I'm a Guy 18 she is 17 we are like family friends, me and her have known each other since we were babies we acturally have baby pictures together even one kissing lol I don't know how we are babies then. Anyway though the years me and her were the greatest friends ever doing everything together. Movies, beach, travel, the gym (me workout her only to keep in shape) and even clubbing. So yesterday was my 18th birthday now I noticed she was dressed in a big shirt and interesting skirt she always dresses up y but that wasn't her. So she hugs me tells me happy birthday and our parents tell us go to my room to hang out and she gave me her gifts, an Ed hardy shirt, the rocky DVD collection and well something unexpected. She said "I have a special gift for you" she took off her skirt and shirt and there was a white bra and a belly dancer skirt and she popped kissed me and whispered "happy birthday baby" And she walk back a couple of steps and started belly dancing really sensual. Slowing something fast moving her hips I was in shock and the best birthday gift o my life but then mom walked ain and (my mom treats her a like a daughter) and started yelling and got really mad saying WTF is this what are you doing watching her dance like that. I'm so confused first why did she give such an intimate birthday gift and two why did my mother get so mad we weren't doing anything bad or ual. Explain please and thank you