alslam alykem my grandmother was sick in the hospital and doctors were trying to give her the medicne and she refused. so i had to give her it by placing an oxgen mask on her face to help her breath. she cant stop taking it because she cant breth on her own. while i was trying to place the mask, she said "may allah punish you" she passed away after a month but i went back to my hometown. i keep feeling guilty because i mad her angry. i keep feeling that she never forgive me although my family told me that she forgive me. since that day, i feel that i am tired, and i cant study or work. i started to have a lot of nightmare. i cant forget the scene when she asked allah to punish me. So i want to know, is what i did was a sin and what can i do thank you

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

You are not sinful for this act which is placing the mask on the face of your grandmother since you did so for her benefit. It might be that it is normal for her to get angry while she is in such a state of sickness. However, if you did so in a harsh manner, then this is an act of cutting ties with kinship; in which case, you are obligated to repent.

In principle, a mother [or a grandmother] is affectionate and caring for her children, and it is believed that she would forgive you. Since your family had informed you that she forgave you, so why are you worried because of the whispers that you mentioned?

It appears that the tiredness and inability to memorize and the nightmares that you are experiencing are all due to your current psychological situation. 

Hence, you should live your life as if nothing had happened. Besides, you should supplicate for your grandmother; for more benefit on being kind and dutiful to the parents after their death, please refer to Fatwa 83233. 

Allaah Knows best.