Casting magic in The Vampire Diaries can take a physical toll on the users body. We see this a lot when Bonnie tries to perform magic beyond her ability, she starts to bleed out of her nose.
In this instance Gran overexerted herself to help Bonnie save her friends, in order to save Bonnies own life. Gran not being a young women anymore and casting magic far beyond what her and Bonnie should have attempted threatened to consume them both, If im remembering the scene correctly i believe that Gran attempted to talk Bonnie out of it, but with Bonnie insistent they rescue them Gran took the brunt of the magical burden, to keep Bonnie from killing herself.
Yes it appears im right according tothe wiki
Bonnie pleads with her grandmother to help break the seal. Sheila tells her that she may not be strong enough but with Bonnie's help, maybe it will work. They both proceed to chant the spell.
Bonnie was insistent that her Gran help her save Elana. wiki