Trim looks fine, but are you working hard to stay in that position? When I'm trimmed correctly, I can relax and remain in the horizontal position. In the past, when I've hired gear that isn't quite right, I've had to arch my back and perform other contortions to get my trim right which could cause back pain . As an aside, I don't generally get back pain, but nearly always have a little sciatica for a day or two after diving.