Yeah I'm thinking it's just core weakness, I notice when I do normal dead my lower core is the part that hurts the most, and it limits the weight I can lift.
I may try to concentrate on some core exercises, having said that I don't know anything that works the core like heavy deads!
Assuming it is a lack of core activation, control and/or strength, your immediate focus shouldn't be on trying to hit the core core
hard, but on trying to make the core do anything at all. Take note of the technique point I gave earlier on drawing everything in and lifting your chest. Practice that a lot, both in and out of the gym. As an exercise, do progressions from quadruped stomach vacuums through to bird dogs, and once you've mastered that, move onto variations of planks, side planks and woodchops.
Quadruped stomach vacuum: Get on all fours. Get your spine neutral -- natural arch in the lumbar, chest up. In the eccentric, while keeping your spine in position, release tension and let your belly sag down towards the floor. In the concentric, control the tension and draw your belly button up into your spine. Again, keep your back position. Eccentric on the inhale, concentric on the exhale, breathe slowly and draw in/sag out slowly.
Bird dogs: From the quadruped position, keeping your spine neutral and your stomach drawn in tight, raise the opposite hand and foot off the ground and reach them as far apart as possible. You will have to start this very slowly, and balance will be an issue, so you may not get full extension at first. Hips and shoulders should remain level at all times.
You can merge between quadruped stomach vacuums and bird dogs by only pointing a leg back without pointing an arm forward (glute kickback) or by pointing an arm forward without moving the opposing leg.
As your skill improves in that Jane Fonda-ish pattern, you'll become better equipped to use your core properly in other exercises and in heavier core exercises, and your ability to deadlift without undue back pain will improve.