How low is the pain in your back ? You need to figure out where the pain is coming from, most common are, Lattisimus dorsi (Lats), top of the glutes (where they connect to hip) or the fascia (common with squats). All are under enormous stress when doing back squats.
Once you figure out where the pain is coming from, start with aggressive mobility work of that muscle. I would do self massage and static pressure followed by stretching (use a roller or ball for your back). I think you will find your back muscles will be super super tight especially if you have been lifting and not doing any mobility work.
Also don't rule out hamstrings, tight hamstrings will reduce hip angle in the squat and can cause many different back problems.
As others suggest reduce the load significantly but don't stop lifting. Keep up the mobility work you will be surprised at the results and you back will be stronger than ever.