You've been doing this for less than a year and you're on a 5x5 with interwoven HIIT? I don't know how you feel but that seems like an awful lot, that would keep anyone torn down pretty hard. I don't think (check anyways though) that 5x5 rotates the way SS does, especially with the lighter day built in on Wed. in the middle. If the DOMS is really bad, I would think about cutting down on the HIIT at least for now so you can focus on adding LBM (assuming you eat clean, which may not be a safe assumption). Then after you finish 5x5 maybe do a cutting cycle?

That'd just be the way I'd do it to get the most of my 5x5 but I'm no expert on that routine.