Cramping Calves

reederx5

I went to the gym last night and did the stationary bike for about 15 minutes and then switched to the treadmill. I walked for two minutes and then started a slow jog 4.0mph and almost immediately my calves were crampy feeling almost like a knot. Is there something I need to do or a way to prevent this? Am I at risk of injuring them if I continue?