I think that I'm feeling something metaphysical?

If you are sure that this is your great grandmother, I'm thinking that she's not around you to do harm to you, she probably just wants to communicate. All you need to do is openly talk to her, even if you don't feel her presence all the time. Ask her what she is doing here; why she needs to contact you. Also, if you have had much experience with a pendulum, you can use it to ask her simple yes/no questions and things like that (Im sure you already know how to though if you've been working with one for quite some time). But always be cautious when using items such as pendulums and Ouija boards for they can often attract or open doors for other evil spirits to enter. I think your best bet is to pray to your great grandma and talk to her that way. You shouldn't feel scared or frightened by her presence because she is family. Good luck!!