Am I mistreating my sis?

I'm not sure why my sis gets so mad at me for this kind of stuff. But, like about 6 days ago my sis sent me an email that literally said "hey, what are you up to today?"and nothing else. Well, I wasn't able to get back to her that day with an email. So like the next day she called me, we talked for about 30 min. Then later on that day and the next day we texted back and forth. So then yesterday, she dropped by to talk for a quick minute and then she brings up the fact that she had emailed me and I never emailed her back. I told her I thought since all she wanted to know in the email was 'whats going on' and we had talked and texted since, that it wasn't important to email back. She had this look on her face like I was repulsive at that point. Then, today she called me three times this morning (within 1 and a half hours) but I had been very busy all day with daughter's school party, paying bills and so when I saw she had called third time ( I was still busy) I made a quick minute to text her (telling her I was sorry hadn't answered earlier but had been busy and would call her later) and she texted back and said she got the point that she was pestering me. Then I called her at 5 pm tonight and she acted all snotty and said she had to go, after just two minutes. I get frustrated because I think I'm doing ok, but she acts this way. I don't know if I'm right or if I'm truly mistreating her.