Since it's only in one leg and just in the calf region...then it seems more likely that it is a pulled/strained muscle.
Many people don't realize what we can do in our sleep and at times can totally jerk, stretch, and strain our arms, legs, neck through the night.
As well as sometimes during the day you can do something that at the time doesn't seem to hurt or strain your leg but then a day or two later it shows up.
The great news is that you do not have a blood clot which that is always the first thing someone should get checked out if they have severe and sudden calf pain.
And yes..it could be a regular fibro nerve pain that is just sending a stronger signal to that area of the left leg.
Whatever it is...since there wasn't any type of accident or fall down the stairs or actual torn ligament/tendon there doesn't seem to be any reason to worry and hope it just calms down in a few weeks.
Just take it easy for the next few weeks with walking a bit more slowly but staying mobile as well as others have mentioned...adding more magnesium and potassium if you don't take this supplement for a few weeks can absolutely help with cramping.
SB and "the pup who snores loudly"
ACDF C5-C7, (no hardware), with autograft bone Nov. 2001
(reabsorption of bone 2 years later...still lost in body..expect to burp it out at anytime..haha"))
ACDF with hardware, allograft bone Nov. 2005
Anterior and Posterior CDF, allograft bone with BMP, removal of old hardware, use of titanium plates, rods, screws, & kitchen sink (lol) Oct 2006