I'm so very sorry @KittysPetJoey
You've suffered a very recent loss, and of course that may leave you feeling bereft. It's a very natural response. Hearing your grandmother's diagnosis must feel like another loss, as perhaps you imagine her path following that of your mother. Try not to look too far ahead, and take each day as it comes. And it's okay to feel sad. Don't be alarmed by that feeling. But also try and do something with your days, have some sense of routine, try to get out.
That really intense grief will pass in time, but just as we're all different, so too is the way we cope with grief. So go at your pace, and remind yourself it's okay to feel sad. But also consider you still have a life to lead, and I imagine your mother would wish that were a happy one for you. And well done for sharing how you feel. I hear you.