mother in law hates me for no reason

My mother in law for some odd reason hates me. I've tried my hardest to get along with her and have her trust in me. She always throws digs at me.. Behind my back. The other day she  made it seem like I'm not paying attention to my husband. He had a headache and I gave him an Advil since he had tea before but that didn't get rid of it until I have him the Advil. She was making t seem like I'm not taking care of him . What else am I suppose to do if tea doesn't work? Later his headache went away because I gave him the Advil and she's still trying to throw digs. I acted like I didn't hear anything and just enjoyed the rest of my evening. On Tuesday she called me and I couldn't pick up because I was at work and didn't have my phone on me either. 
She calls my husband telling him I'm ignoring her . He told her I can't always pickup I'm at work and I don't always have my phone on me. She was like well she needs to have it on her. I called her back when I was done she starts yelling at me why I don't have my phone on me and why am I. Ignoring her ? I told her I couldn't. Pick up she didn't care she was mad . I didn't bother to keep talking to her she doesn't understand . She wanted me to watch over her grandkids  for a while ... I can... But she tells me things last minute and I'm busy. I don't always have free time and I prefer to plan things ahead of time . She got mad at me because I can't do her the favor but she needs to tell me ahead of time and not get mad because I can't . I can't drop everything because if I don't she'll get mad. Unless it's an emergency than yes I understand but these are things she askes me she can tell me ahead of time so I won't make other plans.... Do you talk to her ? Or tell her husband to talk to her ? Seriously idk she gets mad at me for everything!