I run regularly but when I go fast (for between 100 metres and 3 km), I get pain in my calf muscles, right below the knee. This is in both my legs. Why is this?

Unfortunately, I cannot say for sure what is causing this problem. It could be that you have injured the area whilst running, it could even be to do with the way you are running, or it could even be to do with your shoe wear - sometimes they don't offer enough support and this can cause pain.

I would recommend taking some time off from running, perhaps doing a gentler activity in the meantime, and see if the pain improves after this. Also, apply some heat to the area (a warm, damp cloth can work well, or a hot water bottle) to see if this brings relief.

However, if the problem persists, as always, I would get it checked by a doctor or physiotherapist just to rule out any injuries.