Do you think it is possible that I manifested my grandma's death?
That's hard to believe, but why do you feel like you caused it to happen?
Her death just had a lot of bad energy surrounding it.
Even if you feel that you might somehow be responsible for your grandmother's death, you are not. Death is sad, so most deaths feel like they are surrounded with bad energy. You did not manifest her death or make it happen, and perhaps talking through your grief with a therapist would be a help to you.
I think she was just sick and old, don't worry about it.
Exactly! She is just sick and old, so now you are coming to your senses. Don't put the guilt on yourself because you can't control what happens to your grandma.