There's some tension between my mom and me at times. How can I best deal with this?

My mom is elderly and had a mini-stroke in July. Since then, her health (especially her emotional health) is fragile. She had the stroke due to my father having to be placed in a nursing home. Anyway, I see her every day, go do stuff around the house, run errands for her, etc. I'm the only sibling around to help out so she has this lady who comes to do the housework a few times a week. Anyway, I'm trying to encourage my mom to start driving again, etc. because she says she gets lonely. When I tell her that, she says I'm preaching to her. Tonight, I was telling her that of all of the people I know, I'm pretty much the only one who takes care of my parents the way I do (by seeing them every day). She then said that elderly parents should be in heaven because they're a burden. I was actually mad and frustrated after that and I couldn't really say another word for the rest of the night. I don't know if I feel guilty or what; I can't pinpoint it. I'm trying to do my best but sometimes I become overtired and frustrated. Help!