Top Tip

I've had the same problem recently. As far as I know, it can be either because you exercise too much - too much pressure on legs - or might be because you are really skinny and your fat tissue has gotten thinner - e.g. if you've been on a diet for a while.
In the first case, try to make your workouts more relaxed, focus on exercising your upper body more maybe - and less strain on legs.
In the second case, you might have to review your diet, perhaps, include some healthy fats, such as nuts, avocado, etc.

More serious, it may be varicosis. You might have to check with a doctor then.