Jack Straw
I look nothing like him!
If it's a pulled/torn soleus (the lower of the calf muscles, kind of sits under the gastroc), it usually only hurts when running as it's not much used in walking/cycling (I've done mine a few times) so sounds like this might be what you've tweaked. It's also a bit of a sod to heal and can take time. There are some specific soleus stretches you can do (loads on youtube), and if you can it's really worth a session with a sports massage therapist or physio, as (as well as actually getting a correct diagnosis - there are loads of moving parts in your lower leg, it could be something else) even if the tear/pull has healed it's quite likely there will be some scar tissue impinging proper movement of the muscle - this may even be causing the tightness you feel.
For anything that's not obvious or taking it's time to heal, a session with a physio is almost always the best thing to do.....diagnosis, treatment and recovery will usually be far quicker and more sustained
Sorry to hear you've had a similar problem, but it's reassuring for me that you say it does take time to heal and it will eventually go. I've looked on Youtube and found a video that seems to be about exactly what I've got. I've also just ordered a muscle massager too.
Just turn the volume down a bit so their voices are less annoying!
https://www.youtube.com/watch?v=E29xkp-_Ogs