Runners - can anyone help me work out what I've done to my calf?

[Blatherskite]

I've been running for a while now. I worked up slowly to 5k-ish on my own and then joined a running club last September and have been building up slowly, running twice a week with them and Parkruns most Saturdays. Since about last November, I've been doing 1 10k, 1 5k and a technical training run each week.

In the past 3 weeks, I've started training for a half marathon and have been doing longer runs on top of my usual runs, building up by a mile a week. My longest run was 9 miles last Sunday. I felt fine doing it and felt (and looked I was told) as if I could have happily gone further. My knees were a bit tight but I've been doing lots of IT band stretches and they were feeling much, much better.

Last week I pushed it a bit and did a 10k 2 nights running. Both were social runs though so I was never pushing for speed. The first was a bit hilly but more long and drawn out than especially steep.

My legs felt fine all the way through the 2nd of the 10ks. I stopped, stretched and then gently jogged back to the car. As soon as my right foot hit the floor, I had a sharp pain, enough to make me wince, up the inside of my lower leg, definitely in the calf muscle rather than on the bone, right in the middle of the length of the muscle. I continued to jog to the car but it didn't ease. I assumed I'd overdone it and thought nothing more.

I didn't run for the next 2 days and was then due to tailrun at Parkrun. I assumed 2 days of rest would have helped but as soon as I started to jog, the pain came back again. It didn't ease much over the whole of the 5k. Thankfully, I only ran about half of the route and even that was very slow.

The pain stops as soon as I stop running and no other activity brings it on - not walking, not climbing stairs, not standing on tiptoes....

What on earth have I done to myself and how do I fix it? I had to miss yesterday's 10 mile run and I was doing so well!