Problem solved, I rebooted my phone few times and cleared temporary files twice, there's now ~500Mb of them so that's normal I guess.
Thanks anyway! :)
I have this one: http://www.e-ville.com/en/1030-protection/11622-nokia-lumia-925t-flip-cover-4-colors.html
It's ok, but I just bought Qi cover for my 925.