You're sore Wednesday because you squatted Monday. Soreness from lifting can easily last two or three days, and even get worse on later days. It's called Delayed Onset Muscle Soreness or DOMS.
Since waking up this morning, my lower back is very sore. It is as if I did a heavy workout. I don't understand why this happened. This wasn't as sore yesterday.
You did do a heavy workout. On Monday. That's why this happened. It wasn't as sore yesterday because the Onset of Muscle Soreness was Delayed. That's Delayed Onset Muscle Soreness.
Keep squatting. Keep walking and stretching on rest days. Keep an eye on your form. For today's workout, warm up thoroughly but stick to the planned lifts.