I hope someone can answer this expertly, because I get the same problem, on the 2nd toe of each foot (the one next to my big toe) and to a lesser extent on the 3rd (middle) ones after doing a 3-day backpack.
I had the problem with my previous pair of boots, and when I was buying my present ones (which are now worn out and due for replacement) I was told my boots were a fraction too long for my feet, so that my feet were moving forward inside the boots and my toes were striking the front, resulting in the bruising.
At the same time, I can well believe the opposite, that boots that are too tight/short might be the reason.
I don't know if thick layers of socks helps, but I assume it does.
The bruising is no big thing, by the way. The soreness wears off after only a few days, after which you're just left with unsightly dark toenails until they've grown out. if it's really extreme the nail might drop off after a while but it's not going to kill you. I'm guessing maybe you're not too happy about what they'll look like, as you're of the sex that takes more care over their appearance than us blokes.