How to handle my Mom, gets nasty and accusing?

mom has had this mean attitude on and off for awhile. yesterday i worked a 13 hour shift at the hospital , i came home exhausted, when i got home i took mom to the bathroom changed her , walked her back to the tv room made her supper , we stayed up and watched toy story 3 she loves those disney programs . i took her into her room at 10 pm helped her in bed , made sure she had her oxygen made sure she had a cold drink , and i tucked her in and fell into bed. she woke me up at 0330 and told me she was thirsty so i walked her to the tv room asked her if i could take her back to her room she said " no i'll watch tv . so i gave her a cold drink , a ensure and a yougart , said i'm going back to bed. i get up at 0730 and she is mad and nasty yelling at me " where were you ?" were you out with your friends? i said mom what friends ? i was in bed in the next room . she was just nasty calling me names , i told no more i'm done talking to her , she kept on and on , i tried to explain to her i just went to bed, she accused me of being mean and how could i be mean to my own mother. how dare i treat her like this.. she was moving out. i said where mom? my sister died in aug my only helper with her , my brother never calls or even offers help financially or anything. he says " call if you need anything?" oh brother what a crock he was very helpful the first month my sister died , he and my brother inlaw my sister's husband does help me if i ask . which i am grateful . because my brother is useless. oh everyone wants to help the first few months after a death , after that i am alone. my sister always said " it's just you and I helping mom and dad and she was right. i am alone , i have no idea how to handle mom when she gets nasty , explaining dosen't help her understand. instead i just shut up and sit there.. i just want to know how you handle your family member acting all crazy . i'm a RN and haven't a clue. ughhhhhh thank you ..