Taking a chord from the parallel minor of the associated major key [is a thing][1]!

Let's break your song down to simple notes:

• Cm chord - C, Eb, G
• G chord - G, B, D
• Dm chord - D, F, A
• Am chord - A, C, E

The only accidental here is Eb, but we also have B and E, so it's safe to say we're not exactly in Bb major/G minor. Notes C, D, G and A appear in more than just one chord, and they are mostly minor chords so the A minor sound is surely present here.

The Cm chord itself has C and G, so the only anomaly is that Eb. This note can be considered a tension, because the dissonance it creates contributes to it's purpose inside the song. You need that Eb. It's also a nice melody movement from the E in the Am chord. [1]: http://www.wikiwand.com/en/Borrowed_chord

Taking a chord from the parallel minor of the associated major key is a thing!

Let's break your song down to simple notes:

• Cm chord - C, Eb, G
• G chord - G, B, D
• Dm chord - D, F, A
• Am chord - A, C, E

The only accidental here is Eb, but we also have B and E, so it's safe to say we're not exactly in Bb major/G minor. Notes C, D, G and A appear in more than just one chord, and they are mostly minor chords so the A minor sound is surely present here.

The Cm chord itself has C and G, so the only anomaly is that Eb. This note can be considered a tension, because the dissonance it creates contributes to it's purpose inside the song. You need that Eb. It's also a nice melody movement from the E in the Am chord.