A knot in calf
Massaging can help, but you should try stretching of your calf muscle.
Extend your leg out, keep your knee stiff, then point your toes toward your face. Then, step one foot forward and begin leaning your body toward the wall until your elbows are flat against it. Hold the stretch for 10-20 secs and release.
This will stop a charlie horse from happening and may possibly give you some relief now.