the swelling appeared overnight may be secondary to the lymphoma
your mother had. since it is a large and hard mass it is probably a spread of the lymphoma and is of aggressive type. at this stage its difficult to predict the expectancy because it may vary person to person. but theoritically she may have upto 2 years if she is retreated for the lymphoma. retreatment also depends on her health status.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications. I would gladly help you.