Skipping the gym due to lower back pain?

It's possible that you've strained or overused a muscle. In the absolute worst case you have herniated a disc during training, but that tends to hurt a lot from the moment it happens. It's best to get it checked and avoid anything that worsens the pain as much as possible, do not pop a painkiller and keep doing injurious things, it'll hurt like hll afterwards and makes the injury worse. However, back injuries can feel a lot worse than they are and a light injury often heals within a few weeks with most of the pain gone within a few days.

If you have a light injury simply avoid deadlifts and bb squats, etc. (any heavy lift actually), sit and stand with proper posture and be careful with lifting things around the house until the pain has gone and then rehab, start very light and train stabilizer muscles, and make sure you're using perfect form. You should feel your legs a lot more than your back after deadlifts: your back should primarily work as stabilizer not as a lifting muscle. Remember that perfect form and safety go before weight, you may want to videotape yourself to see how you're doing.

It's your leg day, and prob the cumulative effect of several. These things don't necessarily happen instantly unless there's an acute injury, like a major tear.