8 month old black lab puppy gets aggresive when we sit on couch?

she'll be eight months on Wednesday.. she is spayed, and never had this problem before, and i don't know if i can say its really aggresion, but whenever anyone sits on the couch she goes crazy.. barking growling and nipping at your hands feet shoes legs anything really. but her tail is also wagging. we have tried to ignore her but she just keeps biting, we try putting her outside but the next night it is the same deal. she got trained at petsmart and was fine for the first two months after the training now she had this one problem. ive read that it might be her going into adolesence but i don't know how to solve the problem. this is the only biting problem she has Please help!!! Thanks in advance :)