Am I a bad friend ? Advice please ?

So my best friend (more like as sister) called me today asking whether we up for this Saturday. So I asked her what’s happening and she said we were celebrating Valentine’s Day and we were going to a café. So I thought it was the Saturday before Valentine’s Day but she meant the 18 Feb. When I told her that she asked why would she cancel. She did mention to me like 3 week ago she wanted to go out but never mentioned a date.

So I’m in grade 12 and she’s in grade 11 so basically I have 6 months to complete a year’s curriculum and then the remainder of the year is revision and test. So I told her that I was given 8 assignments last week that are due for 25 Feb and I wanted to complete some of them this weekend so that over the week I’m not stressing myself or behind with my other work. The initial due date was 6 March but then they changed it to 25 Feb. So she told me she was disappointed that I couldn’t come considering I knew and stuff. So I just hung up and she hasn’t called. (BTW another friend was invited and she going and now I’m experiencing FOMO)

What do I do?