what causes my calves to tighten up
Calf muscles may have gradually tightened up over a period of months through not stretching enough before and after training. Tiny micro tears in the muscles cause them to go into spasm. When they are in spasm or contracted then blood cannot easily get into them. The muscles have squeezed the blood out like a sponge.
Runners and other athletes are also prone to calf tightness, as the calf muscles are constantly firing to stabilize the ankle and both absorb each landing and push the heel back off the ground. Interestingly, calf tightness can also be symptomatic of weakness elsewhere in the leg. If your glutes and/or hamstrings are weak, your calves will often try to make up for that weakness, which means the muscle gets overused, which in turn exacerbates calf tightness.