I'm glad someone has put this subject up as a couple of year ago I was just 10 minutes into teaching a two hour seminar on Muay Thai and was demonstrating how to kick with the front leg by skipping and bouncing off the ball of the foot. I suddenly felt like I'd been shot in the back of my calf it was so painful and I had to get through almost two hours of showing people how to kick and knee,etc luckily one of my students was there to demonstrate for me!
I must admit that on this occasion I did'nt stretch properly before I started but still stretched for ten minutes at the beginning of my seminar,in all my 30 plus years of kicking bags and Thai pads,etc this was the first time I'd suffered with my calf.
I always have a good 10-15 minute stretch before any form of training but I do find that even when we all warm up and stretch that things can still go wrong.
One of my students has just started having problems with his calf from kicking the Thai pads so the advice given by 'Kelly' above will be very useful for now and in the future.
Luckily for me my calf injury was'nt bad as yours 'ashilto' but I could'nt walk too good for weeks and was out of action with kicks for about 2-3 months,since then my kicking has been back to normal so I wish you a speedy recovery!...Phil