Why do my lower calves seize up so much when I run now

tight hip flexors / forward pelvic tilt, insufficient warm up + cool down, electrolyte deficiency are probably top 3 possibilities

Sound response here from Charli. As a DPT I’d recommend also considering running movement quality and ensuring good range of motion and strength throughout the entire lower body.

Also when you think about things from a load management standpoint it’s important that you follow the principles of proper training - progressive overload and making sure you are starting at a training volume you can positively adapt to