Yeah, that certainly sounds like you have a lot of problems there. Hm, let's start by ruling out the usual suspects.
Try deactivating all plugins except the one sitemap plugin you are using. If that resolves the issue (you may need to clear your browser's cache too), reactivate each one individually until you find the cause.
If that does not resolve the issue, try switching to the Twenty Fourteen theme to rule-out a theme-specific issue (theme functions can interfere like plugins).
I suspect it might be the theme, but good to start with plugins just in case.
