I'm not sure when this problem started, but it is at least a couple of weeks old. On deadlift days, right after the first warm-up set (on really low weights), I get a kind of tired feeling in my lower back, with soreness - not pain - upon standing up or holding a weight in front of the body. Also, I get a heavy lumbar lordosis and tight erectors, and all this from warm-up.
I actually manage the heavy sets without problem and with much less of the desribed feeling, but when I try to do Romanian deadlifts after the heavy deadlift sets, the lower back feels really sore and I can't make more than a few repetitons, despite the weight not being high. Thus, I've had to stop doing RDLs for a few weeks now, which annoys me. Any ideas of what I might do to solve this? A reference video of med doing a heavy set is attached.