You are doing the right thing by elevating her leg.
What is happening is that there is too much fluid in her leg and it is weeping the fluid.
Loosely wrap her leg in a clean towel.
It might do this for a while.
If she seems short of breath you need to get her to the ER as she may be getting too much fluid build up in her lungs also.
I would call her doctor and tell him/her what is happening.
If possible get your moms weight and have that ready and if she has had a rapid weight gain.
Let me know if you have more questions.