I am starting out doing the 5/3/1 and after a couple sessions deadlifting, I am experiencing some pain in my lower back. I've never deadlifted before so I am not sure if my form is correct or if my lower back muscles are weak (I sit at a desk all day). I am able to do all my assistance exercises after that without any pain, but bending over and sitting in some positions is uncomfortable. I am thinking of backing off the weight to make sure form is correct and build those muscles. Looking for a suggested starting weight? The past cycle I used 250 as a training max. Thanks ahead of time.
Next time you deadlift record it from the side so that you can see what you look like. Sometimes it’s pretty obvious and easy to fix but If it looks fine to you then post it online and we’ll critique it.
I’m admittedly a beginner as well, but I would suggest practicing proper abdominal bracing as it helped me immensely with reducing lower-back strain. I found this video of Chris Duffin coaching abdominal bracing to be incredibly useful, maybe it will help you too:
On top of the above, I would revisit your chair at work if you are a desk jockey, like myself.
I sit on an inflatable ball and keep a couple of tennis balls to roll my feet and lower back during the day. Makes a huge difference as well as doing that mobility work Jim tells you to do.