Why did my friend get so pissy with me?

She’s hardly ever in touch and then contacts me, saying she’ll ring me in the morning, which works for me. Then later, she says would the afternoon work for me. The afternoon doesn’t suit me, as I’m considering attending choir and will have to leave in the afternoon. Then when I explained this and suggested Saturday, since she’s got work and I’ve got choir, she got really offended and sulky/snitty. I’m genuinely confused. We both have Asperger’s.

Why did my friend get so pissy with me?
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