feels like theres so much pressure on my shin bones?

This seems to happen to me every now and again, maybe once or twice a year at the most.

And completely out of the blue.

I'm pretty healthy, i walk a bit so am quite active, but not excessive.

I'm about average weight and height.

I'm 20, about 5'10' and weigh about 12 stone.

It just feels like there's so much pressure on my shin bones, but then most days it's fine.

I dont have weak bones or anything either.

Any ideas?