Your pup sounds like she is really enjoying training, which is great.
What is happening is your reinforcer is too distracting for a young pup and you're going for too long for a 12 wk old pup.
Find a treat that is good, not GREAT (too distracting) and keep sessions less then 5 mins, she's telling you that 5 mins is her limit.
My border collie pup could only do 5 min sessions until about 5-6 months before she could actually concentrate on the task at hand. Pups are all over the place, as they should, they're taking in the world!!!
Have fun with it! Try to call it quits before she does, leave her always wanting more training!