Lumbar doms can be very uncomfortable. If it's just muscle soreness and it goes away after a couple days, there is no cause for concern. You are using your entire back when you deadlift, especially during a conventional deadlift. Back fatigue and soreness can be expected.
As far as your form, it looks decent. One thing I noticed was that your feet are inside your knees. If this works for you, then great. Personally, I like to have my feet about hip-width so that all my joints are basically in a straight line perpendicular to the floor. I feel like this really helps me generate leg drive and break the bar off the floor with explosive force.