Sorry about the delay in writing back to you.
There are a few reasons that could explain why Jetpack considered your site down while it was in fact accessible when you visited it. The most likely explanation is that your site was up but loaded too slowly, or that your hosting provider blocked our monitoring agent from accessing your sites.
Jetpack tries to access your site every 5 minutes. When Jetpack’s Monitoring agent notices that your site is down (or can’t be loaded in less than 10 seconds), it tries to load your site again twice from that server location. If that fails too, it tries again from 2 other locations on 2 different continents, with timeouts of 20 seconds this time. If these 2 fail, we consider the site as down and send you an email. By that point, I think it’s fair to consider that either there is a problem with your site, or a problem with how your server handles our monitoring agent.
The Monitor emails you received should include some extra information to help you figure this all out.
You can also find more information about Monitor here:
https://jetpack.me/support/monitor/
If the frequency of notices is bothersome, you can always disable that feature here:
https://wordpress.com/settings/security/
I hope this clarifies things a bit.
