Lower back pain

Hi all, I did a sprint last year and didn’t really train. This year I’m doing another sprint on my way to an Olympic. My lower back is hurting though. I took a week off of training after I got sick, have been stretching a lot, and it sometimes feels like it’s getting better but other times it seems like I’m making no progress. It hurts most after my run days. Anyone experience anything like this? What did you do? What advice do you have?