User blog comment:B214/Volume 21 info & spoilers/@comment-71.205.30.36-20160407022108/@comment-82.199.211.146-20160408154400

I was thinking around the same lines, though since it was mentioned that with DD, Issei has reached HD class, I think that the power of "infinity" he got from Ophis, filled him with as much power as he could potentially have in CC, meaning Ddraig's prime power. Since there has to be a limit to how much power he has in this form, otherwise he would just become prime Ophis. And the pain is because his body contained power beyond his physical limits. And the nerfing just includes having Ophis regulate how much power she provides him.