Lump on calf?

[phil_h_88]

Evening, I'm just after a bit of advice about a lump I've found on my calf. I'm not sure how long it's been there but I only noticed it today. It's on the back of my right calf about 4/5 inches below the back of my knee. It's painful and tender to touch and feels about the size of a 5p coin. I run regularly and train legs every week.
Anyone had anything similar ? I'm just wondering if it could be a muscle knot/tear or something more serious.
Cheers for any help