Strain Update
Left side is fine.
Abit achy on right side.

The strained muscle seems to be Serratus Posterior Inferior Muscle
YouTube

It's painful/tender when I press on the muscle. Typical strain symptoms.

Prolly busted this one while doing free-weight RDLs last week.


Moral
Training on smith machine/machines really can cause imbalances due to lack of stabilizer muscle recruitment.