I second investigating the pack as the cause, before you do anything. I have had a similar lower back issue that was definitely related to the type of waist pack I was wearing while nordic skiing. Yes, I already had a sensitive spot/issue there, but the weight and angle of the pack just pushed it over the edge.
I've spent 2 months rehabbing, and while it's better, and I stopped the PT visits, I feel like this is something I could have avoided. Now I have to do a whole bunch of stretching to keep it from coming back! I will definitely be investigating new options for next winter.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport