Lower calf suddenly got tight during run

This has me baffled. This morning I went for a run; warmed up with one mile, stretched and then set out for an easy paced 60 minute run. Just 7 minutes into it my lower calf tightened up, literally from one step to the next. It wasn't a cramp, but felt like something just waiting to tear, so I stopped and walked home.My first thought was maybe my hydration was off, but I had barely begun to sweat. Now I'm wondering if it could have something to do with the one mile open water swim I participated in on Sunday. I put in a max effort, pulling and kicking much stronger than in any previous swim. Could my lower leg have cramped a little while swimming and that's why it flared up today?anyone have some thoughts?