Was I to blame for my elderly mother's sudden hostile behavior?

First, to give some background info. My parents had me late in life... I was the "oops" baby. My sister and brothers are much older me. I am the only child that remained in the same town as my parents.

I first became a caregiver to my father. He was in his mid-70s when he passed away (in 2003). He had congestive heart failure. In the 2-3 yrs before he died, I was his primary caregiver, even though my mother lived in the home with him. I was married... had my own family. I worked fulltime. I checked on him in the mornings, during my lunch, and after work. She often took long vacations with her friends... usually when he was at his worst. During those times, I was with him nearly 24/7. He would get depressed quite often as my mother would tell him on many occasions that he was a nuisance because he couldn't take care of himself. My dad had a "troubled past"... in his younger years, he wasn't the best husband and parent to my other siblings. According to them, he was abusive and drank alot. I had figured that this was the reason behind my mother's behavior back then... why she was so mean to him when he was so helpless... it was her revenge. He was never mean to me... quite frankly, he spoiled me rotten! So I felt much anger and bitterness towards my mother. I was convinced that she had tampered with his medications and that's why he died. My dad knew she wanted him gone. She did not shed a tear at his funeral. It took a bit of time, but I did eventually put those hard feelings aside, and have spent the years since my dad's death looking after my mother. She's never needed the care that my dad had needed as she is pretty active for her age. The only major help she's needed is in moving from apartment to apartment (6-7 times since 2004). And she has been good to me over the years... a huge help financially... giving me money here and there. She has also been the main source of income for my son, who is in college. She has paid for all his living expenses. I've always told her how much I've appreciated her help over the years... a lot of what I have now is because she helped me get it. I have a strained relationship with my three siblings, I'm guessing because I was always considered the spoiled baby of the family.

Recently, my mother decided to move to a different apartment AGAIN. When she told me she was considering the move, I told her it was her decision, but I would not be able to help her this time. I told her she had two strong boys who could help her. Neither of them helped much with her prior moves. Not only was I burnt out from all the prior moving, I also had many vacations and other events planned during the month she wanted to move. I told her I would do what I could when I was available. She said she understood and she would get other people to help. One brother came to help mid-month and my other brother was set to come at the end of the month. She did a lot of packing herself. I went over after work on occasion and on the weekends I was in town. A few days before the big final move, I stopped by to see her after work... I wanted to see what else I could do to help. To my shock, she started yelling at me and physically attacked me. I couldn't believe that my 82 yr old mother was acting like this!! She called me an ungrateful little b*tch... called my husband a rotten bast*rd. She gave me an itemized list of everything she's given me from the time my son was a small child to recently... and told me she wants paid back everything or she'll take me to court. I don't know what triggered this outburst. I talked to her the night before the outburst and she was fine... I told her I'd be over after work to help her. She said "ok, see you then." She has told me she never wants to see me again. I am not sure how to handle this. Do I go over to her apartment and see if she'll talk to me? Do I give her what she wants, which is never to see me again? I'm at a total loss. I've always been there for her, up until recently. And she knew I wouldn't be available this time. I don't know if her friends said something to get her fired up... or maybe my brothers complained about my lack of help? I'm assuming it has to do with my lack of help, but I can't be certain as she wouldn't tell me why she's mad. She just kept yelling at me for how ungrateful I am. A part of me is hurt, but the other part of me is very angry. The resentment I tucked away years ago has flared up again. Even though she has been a very giving person, I am not sure if I want to see HER again. Does anyone have suggestions?