When I started my lower back would kill me, I already have lower back problems, so I went to the bike shop and the mechanic told me to "ride it out". A few rides later the back pain went away. I understand this is not you starting out fresh but it is your first of the year so at least 4 months off is kinda like starting over. When I took 3 months off my back hurt just like when I started but lasted less in duration. Take it easy and relaxed but eventually it should subside I am guessing.