If someone who you love has died and they appear to you in a dream, the dream is usually VERY vivid. Almost like you're awake and the person is standing right there. You can almost FEEL their presence.
Dreaming about your Grandma is perfectly normal. It may be that your dreams are expressing YOUR concerns about whether she is alright or not. Do you have a particular belief or faith that is of support? If not, it may be that you are a little unsure of where her self, her soul is...
Trust me, you will know if you have a "Vivid Dream", the ones where someone on the Other Side of life are giving you a message or reassurance!!