Lower back pain from picking someone up?

Unfortunately my grandma sometimes falls when she goes to the bathroom at night and since I live alone with her I have to pick her up. I usually do this by lifting her from underneath the armpits.

Trouble is that I’m a small woman, I weigh about 110 pounds and she is around 120. Every morning after this happens I’ll wake up with really bad lower back pain which will be acute for about a week and then mild in and out indefinitely.

It’s almost impossible to sit down for extended periods of time, walking brings relief. But I definitely can’t exercise or do squats because of this pain.

Any tips on how to lift her without this happening? And also how to treat this?