Her stomach hurts so much that she can't stand up,she says she burns from inside to outside,her feet hurts so much that nobody can touch them,she can't even cover her feet. Her mouth is swollen in the morning when she wakes until 10am.she has been in the hospital for4 days but she is not getting better because the doctors don't know what it is.

It is impossible for anyone to diagnose on the Internet without seeing your Grandmother. Apparently Physicians who have seen her can't figure it out.

It often takes several days for test results to come back. Perhaps that is what the Doctors are waiting for.

And it may well be that the may know and "assuming"you a younger child feel it is better not to tell you.

Also depending on her age, it may be several separate things.
She needs to be with her doctors and allow them to find out