FranCat's reply would be the logical choice. I don't mean to question you, but you are sure you're completely logging in, correct? I know sometimes either I won't completely log in, or I'll navigate away from the page and it will log me off. I guess otherwise I don't know what to say. The only other thing I can think of is to make sure you validated your email account on this site, because I had trouble posting after switching email accounts.
I hope you figure it out!