Thank you for the query.
Always when the calf pain appears, veins thrombosis
should be ruled out. Especially if the calf is tender to touch, gets hard and swollen (compare it to the other leg). You can perform simple test. Extend your knee fully, and flex your foot down (it is called Homans sign). If this causes pain, thrombosis is more possible (however lack of it does not rule it out).
Other reason of such pain can be sciatica
due to spine disc herniation. With this condition, pain in the back can be also present. To check for this condition, lie down with your legs straight and ask someone to rise your leg (right and left separately). If you find it painful in the one leg only when the straight leg is at an angle of between 30 and 70 degrees, sciatica it is.
Veins insufficiency is also possible especially if your leg gets swollen while long standing.
I suggest you to visit your doctor, have blood work and D-dimers checked and if elevated, Dopplers veins ultrasound
should be performed (this will rule out thrombosis). For spine diseases you should visit orthopedist and have spine MRI
Hope this will help.