Contacted an ex online after 13 years, need advice

I looked up an ex on Facebook after 13 years of breakup back in mid November and sent an email. She contacted me within 90 minutes and we real timed emailed back and forth for about 30 minutes catching up etc. By the end of November, we had emailed 2 more times, but not in real time. At the end of November, I emailed if she would like to talk on the phone instead and she emailed back to give her my phone number. A few days later, beginning of December, on a Saturday morning, she emailed me to see if I was going to be around for the weekend and what would be a good time to call. I emailed back on Sunday morning to call me any time early Sunday evening. I didn't hear back, so 11 days later, I emailed saying just checking in didn't hear from you. She emailed me 3 days later apologizing stating that her uncle and nephew had Covid and she was also busy with XMAS stuff and asked how I was doing. I emailed back I was home if she wanted to call me and then never heard from her again. What happened? I was hoping to be friends or just talk. Why email me then never follow up. Now I feel like a fool and an idiot. Would appreciate any feedback. Thanks

Just a guess - she's actually in a relationship, was thinking about cheating, decided not to.

It's your ex's actions that you have to look at, you're not a priority otherwise she would have followed through with the arrangement. She's done with catching up with you by using email even though you'd organised to speak to each other. You may feel like a idiot & fool but why bother beating yourself up because of someone else's actions?

You're not an idiot and a fool at all. You reached out to say hello to someone you care about. That's nice. She cared enough to respond. But she has decided she doesn't want to take things further. Some people just want to say hello how are you and then go back to leaving the past in the past. It would have been better if she had let you know this, though, rather than essentially ghosting you.