I'm being locked out, you know? There's definitely something shady going on.

Literal

I [topic-は] locked-out-[passive progressive-されてる][explanatory emphatic-んだぞ]? Behind-dark thing [subject-が] exist [certainly-に決まってる].

締め出される 'to be locked out/shut out' — passive of 締め出す. んだぞ is an emphatic masculine explanatory ending. 後ろ暗い 'guilty, shady' (literally 'dark behind') describes something someone wants to hide. ~に決まっている 'it's certain that, there's no doubt' expresses strong conviction.