This exact same thing has happened to me as well and it sucks because I switched over to https because my traffic was down, so it went even further down..
I did some research into it and found that this is actually quite normal - at least for the short term.
Are all of your pages secure with the green lock or do some have the yellow (amber?) unlocked lock? If so, you have to make sure everything that loads on your page is also https. This includes ALL images (If you are loading from a source that is not https, your page will not be). This also include IFRAMES.
I am currently going through 3600+ different posts and manually switching the images, it's such a huge fucking pain in the shitter let me tell you, I can't do a search and replace because they are all unique and instead of loading from the source I am loading off my website since it's https. Everything is nice after I do the edit.
Some other things to consider.
Did you set up 301 redirects AT THE SERVER LEVEL in the .htaccess file? This is most important from what I read. Much better doing this than tons of individual 301 redirects.
Why do you need 301 redirects?? Because http and https are two different websites, at least to search engines. They will see it as duplicate content and give a slight penalty unless you 301.
I used this code in the .htaccess file. I have no knowledge of Apache or .htaccess but after researching this shit I found this and it works fantastic. It redirects http to https and also www to non-www, all in one
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
RewriteBase /
RewriteCond %{HTTP_HOST} ^www\.(.*)$ [NC]
RewriteRule ^(.*)$ http://%1/$1 [R=301,L]
Again, not 100% sure if this is the proper way but from my research into it, it seems to be. Also, it works.
http, https, www . yoursite.com and yoursite . com are all different websites in the eyes of search engines, so unless you set it up 301's they will see it as duplicate content. On top of that, if you use WordPress it will also duplicate content up to 6 times per post. 6 x 4 = 24 duplicates of every post of content. YOAST SEO plugin helps with this greatly with the extra 6 WordPress dupes and lots more, plus it's free.
Anyways, from what I've seen people say you WILL see a drop in traffic even if you set up everything 100% perfectly as it takes a while for the search engines to adjust. Some people say days, some say weeks, some say a month or two.
I hope some of what I wrote is helpful,
Good luck
