Samantha,
You aren't alone. I am an only child too and was the primary caregiver to my father as he battled lung cancer a few years ago - so I truly know how scarey the situation can be. I remember being in shock with dad. Being an only child and caring for a parent plus your family is a lot to put on your shoulders. Go ahead and call the oncologist's office and ask for the social service contact (most usually have one) - this person is an angel on earth. There are so many resources available to help our parents. I wasn't aware of many of these agencies and volunteers until after I made myself sick by trying to everything for dad (plus kids, husband & work). You'll be surprised by the support that is available to your mom. Take things one day at a time. Cancer is such a big scarey thing for anyone and no one wants to see a precious loved one in pain. When my husband was diagnosed, a dear friend told me that "Faith is a mighty and powerful sword". What I have learned in the past six months is that the sword gets heavy and you have to allow others to help you hold on to that faith.

Hang in there - and remember you aren't alone. Keep us updated on your mom's health.