Are you looking at the right property?
I say this because the old http version of your site I would expect to see a drop as google transfers your site over to https. Have you also redirected all old http traffic to the new site?
I say this because if you have any links pointing to the old version then they will be wasted unless they are redirected and that could drop your rankings.
Where possible I would get any links changed but a redirect will fix any that you cant or miss.
Also if within your site you have any absolute internal links make sure they are all changed as well.