Somehow I've hurt my lower back twice in the last six to eight weeks while benching. Nothing serious, but tightness and pain that made me lay off deadlifts and front squats for a week each time, and I've been making good progress in both lately so it's frustrating. I've always benched with an arch, and never really had this problem before. And it's not like I'm throwing around big weight on the bench right now (235 for sets of 3 last week). I don't raise my ass off the bench, and I'm usually good about keeping the scapula pulled together. Any suggestions on how to avoid this going forward?
You don't need all that extreme of an arch benching weights that light. Just a light arch to get into proper anatomical position will do; no need to arch like each rep is a competition max.
If your arch is the cause of your back pain, figure out what you're doing wrong and fix it.
But, generally speaking, every single rep should be set up the same way as your competition press. Practicing your arch will help you get a bigger one over time, and make you less likely to let it drop.
Arching during the bench doesn't normally cause back problems, but it can. I would guess an anterior derangement (bulge) or a spondylolysis. These injuries don't tolerate much extension(arching of the low back). If this is the case, you may have to lessen the arch.
It is your low back, yeah?
Lay off the hyper extension of you back for a bit. Strengthen the abs isometrically [no flexion of the back] using l-sits, reverse GM's [ab pull downs], plank variants etc.
That's pretty much the price of a big arch and good leg drive. I crank my back benching all the time. Fortunately, deadlifting bends it back the other way.
I had this problem when I started arching. I would work on thoracic mobility. And also focus on breathing into your belly and bracing your abs.