Given the fact that you exercise frequently, I think the most likely cause is that you have a strain or sprain of the muscle. This may have occurred even without you noticing any obvious injury
. You may want to try some gentle stretching exercises, as well as some anti inflammatory medications, and see if that makes a difference. If not, you may want to back off on your exercise regimen, at least for a while, until things start to improve.
The other possible cause that one wouldn't want to miss would be a blood clot in the leg. This is unusual in people who are as physically active as you, but it could occur, especially if you recently found yourself sitting for a long period of time (for example on a long plane flight). Symptoms of a blood clot in the leg, other than pain, including swelling or having one calf appear larger than the other. If you notice this, this is a medical emergency and you would need to see medical help immediately. Similarly, if you have any sudden shortness of breath, chest pain, or palpitations these might be signs of a moving blood clot and you need to seek emergency help.