Hi Jenn,
No one should have to be in such terrible pain, even those suffering from Stage IV breast cancer. My advice would be to take her to the ER ASAP. She doesn't deserve to live this way; she needs to have some relief from the pain, even if it makes her a bit 'out of it'. The ER docs will evaluate her and she will get the help she needs; good luck. She will have to count on you to do this for her since she is unable to do it for herself. She may be in too much pain to appreciate what a good daughter-in-law you are to care so much for her and so I will take this opportunity to do it for her "Thank you, Jenn, for being a good caretaker for your mother-in-law and a good person."
Joan L