Iron levels??

Hi everyone! I ran a half marathon about 3 weeks ago and took about a week off afterwards. Since I started running again it has been awful and I have just not felt like myself while running at all. For context, I was doing most of my long runs while training for the half around an 8:50 pace, and for the half ended up finishing with an 8:38 pace. Since then, I have been struggling to even finish a 4 mile run at a 10:30 pace, it literally feels impossible and my body feels weak. 

Has anyone experienced this?? Could it be due to the iron levels? I know it can take time to bounce back from a race but I can just tell something is wrong by how difficult even such short, slow distances are for me right now. I am starting training for Chicago soon so I want to figure it out asap, any advice or input is appreciated!