pain in calf

I’m having some pain in one of my calves, the position is almost in the center of it.

It’s not there all the time, it comes when I’m doing some movements.

I can simulate the pain when I’m sitting with my knees on the ground and when i’m going with my butt to my heels. Don’t feel pain when I’m jumping or doing similar movements.

any idea what it could be?

edit
must add that i’m having also some slight problems with my achilles lately

Tight calves alway eventually result in achilles inflammation if the calf tightness isn’t addressed. Try to get as much treatment as you can to loosen the area, even self-massage, as this is an area you can reach easily. Pulsing EMS and accupuncture are very helpful as well.