Several years ago one of our dogs was acting very strange and we took her to the vet. She was diagnosed with a brain tumor and she was able to live happily for several years before she eventually passed =(

I googled around and found this link:
http://www.expertvet.com/articles/brain-tumor

It might help you learn a bit about the signs and what some of the treatments can be. I hope this helps! Happy thoughts and lots of love!