Won't Stay Logged In

A couple of days ago I got an email from a user saying that after logging in when they go from their profile page to the site itself they get logged out. When they log in again they go back to their profile page and if they go to the site they are logged out again.

I also noticed that a user can’t log in to leave a comment on a post. When you try to log in from the link in the comment box you only end up back at the top of the page when you try to log in. So I disabled the “must be logged in to comment” checkbox so people can comment.

I have made no changes to the site since its initial setup. It worked perfectly before and then this problem started.

My site is located at https://pet-quest.com

Anybody know what is going on?