Can my Grandmother feel pain, or any other feelings of dehydration and convulsions, while alert?

We need more information. Are you dealing with an elder LO on Hospice care, no longer eating and drinking?
What "convulsions" are happening, and why?

What do you mean by convulsions? Is there epilepsy? Or are the convulsions more like extreme restlessness? Anyone can feel pain whether alert or not. But at the end of life there are many things that can appear as pain but are more restlessness or signs the body is shutting down. As for dehydration, it is painful. Both of my parents were given minimal hydration at the final times of their live after advice from doctors and hospice nurses. We were told that dehydration is a painful way to die. With my dad I gave him very small amounts of water and ice chips. He was much more comfortable receiving the hydration

I'm not sure I'm understanding your question, but since your question is under "end of life," if you're asking if your grandmother can feel pain while in the dying process, the answer is yes. But you would know if she's in pain, as she would be hollering out, and her facial expressions would let you know as well. That is why hospice provides morphine or other narcotics at end of life to keep the person comfortable. I'm hoping you have hospice involved for her now, as they will try their best to keep her pain free. Best wishes.