Google officially says this bro:
"If you see a sudden drop in the number of indexed pages, it might mean that your server is down or overloaded, or that Google is having trouble accessing your content."
"Spikes or dips that appear in several charts can indicate problems with site configuration, redirects, or security."
If you recently migrated from 'http' to 'https' or been experimenting with that, then it's quite normal. Your traffic wasn't affected much right?