I had sex without removing my tampoon. And I haven't had a period since. What could be going on?

That's a tough one! I doubt that having sex with tampon in place had anything in particular with you not having a period. I presume you could have an infection with delayin menses or maybe you just had one of those months where you skip. Pregnance is always the ultimate cause of no period. Get a pregnancy test. See gyn if there continues to be a problem.