There are least half a dozen potential sources of low back pain - piriformis, bulging disc (esp l4/5, l5/s1), sacroiliac, poor core strength, weak glutes, weak hamstrings, tight hip flexors, spondylolisthesis, etc., etc., etc.) . I think it's likely that you'll get different answers depending on whom you ask, but you can test the answers.

Sure, check bike fit. But check with a couple of pros, too - MD, OD is probably better than an MD, chiropractor, Physical Therapist. Check the web, too. A few searches on lower back pain will get you lots and lots of Facebook infomercials, many of which actually make sense. The treatment is likely to be some sort of physical therapy, which will work if you find someone who knows how to fix lower back pain and won't work if you don't.

But consult some pros. A net-based pro will recommend consulting a pro IRL.

BTDT.