Um, is it your erectors? I did something vaguely similar and it was my quadratus lumborum. It's easy to use those rather than something else (hammies) if your back rounds at the wrong time.
Get it looked at. What helped me on the comeback is a couple of things. Again, I might be full of it, but it all seemed to help.
Remember that for squats (front, overhead, back) they are essentially isometric for the abs and lower back. You should also be very aware of moving the hips. It seems that when I get sloppy and use my knees more that I compensate with my back. This is probably my own issue, but I pass it along as information.
- Load up the bar with a ton of weight (more than you can lift), get under it and do some isometric pushing. Yes isometric, the bar shouldn't budge. Do it with excellent form. Concentrate on keep the back flat. Ramp it up (i.e. force of the contractions, say, 60%, 75%, 85%, 100%) gradually. Do 3 or 4 of those with a fair amount of time between.
Use these as a diagnostic to see how everything works and how the power is coming through you from your legs and hips. If anything hurts, back off asap. Iso work is about as safe as it gets.
IF that seemed to work (so no back pain), start your workout and do up to what you did for weight last time. After you are sure you are doing everything ok (again form, form, form. Think flat back, look at the ceiling and at least at this point focus on only moving hips and knees).
If that works, put more weight on the bar (Maybe 50 lbs. more. You'll have to decide that.) Now get under the bar and do your isometric lift again. This time, the bar should float right up. If not, then you aren't doing this lift today. Do a few with good form -- partials might be a good idea, just to increase confidence so you'll know you won't get squished.
Repeat (maybe next time?) with more weight until you get to your load.
Note that I worked in some way to get feedback (iso lift) before really doing anything. If it feels funky doing those it's a good bet you shouldn't be doing anything else.
And I could be wrong. But you'll tell me about that, won't you?