Is it normal that i didn't offer to help my friend with her homework?

A good friend of mine is mad that I am not offering to help her with homework she is struggling with.
I work a full-time job and I’m taking online courses to better my skills to get a better job position, so I have a full plate.

My friend has been in school for 3 years and I have corrected many papers of hers, helped her with her math and geography classes. When I could help her, I do. I’ve also been upfront when I can’t.
The other day, she was venting not understanding her assignment. I asked her if she spoke to the professor. She did, but it didn’t help her. I asked her how about her classmates, but she said they couldn’t help. I told her that looks like she'll have to rely on good old youtube.
She never responded.
Few days later, I asked her how did the assignment go. She said she struggled, but good thing her sister offered to help. Her sister is the only one that ALWAYS offers to help, so she doesn’t have to rely on youtube.
I thought that comment was passive-aggressive, so I asked her if she’s mad at me for not offering to help her. She is. She thought I was making fun of her for suggesting youtube.

I told her I wasn't making fun of her because youtube has always been there during clutch times for me when I was in school. She never responded back. I don't feel it's fair to get mad at me because I didn't offer to look at her homework. I sincerely thought she was venting. Or am I a bad friend for not offering?