My mom had the same thing. I'm so sorry to hear that. Everyone is different but this was my mom's experience:
She was diagnosed with breast cancer and had a lump removed. 4 lymph nodes tested positive. She then went through chemo (not the strongest, didn't lost her hair) and radiation. She was fine for 2 years and then found out it had metastasized to several areas (spine, arms, legs, lungs, kidney). Again she started chemo and radiation. A year later they found a spot on her brain, just behind her right eye. That's when things really began to change. She didn't act herself at times. She always knew who we were but she had a hard time figuring things out. She passed away 4 months after they found the spot on her brain. She was up and around until the last month when she became bedridden. She was still aware of everything and knew all of us the day before she died.
Like I said, everyone is different and this is my mom's story. I hope the best for yours. My heart goes out to you. Spend as much time as you can with her and tell her everything you feel in your heart.