Newbie – Log-In Problem

WP emailed me a Password. I used the link in the email and successfully logged in. My ID name even appears on the WP page, and I can access my Profile. Then I went to Themes and found a theme I liked. WP wants me to log-in again. I tried my Email Address and the Password WP emailed me. A pop-up page claims my email address doesn’t exist. So, I try my ID Name and the Password – and WP says my account doesn’t exist. I go back to the Email I was sent – and using the SAME email address and password – I successful log-in. Very confused Newbie. Please help.