Error 503 Maximum threads for service reached

Hi I have recently started using Cloudflare and added my site to it. Today while browsing on mobile device I noticed that suddenly an error “503 Maximum threads for service reached” started displaying.
This was happening only on the mobile browser and not on Desktop.

The issue was there for around 15 minutes.

Any ideas as to what went wrong. This was around 14:45 IST to 15:00 IST.
Was there any service outage during this period ?

That error message doesn’t appear to be a Cloudflare error. Was a Cloudflare branded error page returned? A quick Google search returns that error message as possible one for Fastly…