Why is my puppy being aggressive

I have a 15陆 week old female Lhasa Apso. She sometimes nips and bites, which I try to distract her from doing by giving her a chew toy or a squirt with a water bottle. While that often works, sometimes she just gets crazy. I attribute that to her being tired, as it is worse at night and when she goes in her crate she sleeps pretty quickly. I am taking her to dog obedience school and that is going well. When she gets in a spat of aggressiveness, she will not let me touch her, she will "back off" (not run away) and then snap at me and if I turn my back on her she goes for the pants and growls loudly. I walk her 3 times a day and play with her and practise training... she is 90% potty trained (thanks to crate-training from the time I brought her home at 8 weeks). Am I overstimulating her? Or might there be a medical explanation for her aggressiveness? We go to the vet next week so I will be sure to ask. I also want to point out that she is not aggressive or possessive of food and or toys.

Well some of that is just the fact that she's a puppy, I have a 12 wk chocolate lab and she just goes into 'crazy spurts' for about 10-15 mins where she bites/barks/runs full speed around the house, etc.

She is also probably testing her boundries and seeing if you are really in charge. If our pup gets too out of control, I gently (but with enough force to know Im in control and mean business) put her on her side and restrain her.