Confused about this conjugation of 忘れる


In KICK BACK, there is a line of the song that goes なんか忘れちゃってんだ. I'm confused
specifically about the 忘れちゃってんだ. I haven't really seen this sort of
conjugation before. I'm thinking it's possibly 忘れる + ちゃ + んだ, the latter two
being connected by て. If that's the case, it seems to express realization on
top of must (ちゃ), as in "must have forgotten...". Could be entirely wrong, I
don't even know if the ちゃ in ちゃいけない can be used alone/in a positive manner.
Any help is appreciated!

