I have 6 strange bruises on my leg?!

Okay here's the deal.

Yesterday I looked at the back of my right leg and realised I had 5 small bruises vertically arranged along my calf, all about 1 inch apart.

I thought nothing of them, but today I noticed a bigger, deep purple bruise has appeared just below the crease of my knee.

I know it's always best to go to a doctor, but should I be worried about these bruises, and what do you think might have caused them?

And no, I haven't crashed into anything.

I am not an active person (I hardly ever walk anywhere), but for the last 4 days in a row, I have been on long walks and come back with really tired legs. I also had to run from a loose dog recently and it really made my legs hurt. Could these bruises be caused by the impact of my feet on the pavement, since I'm not used to using my legs so extensively?

I have never had bruising like this before.

Or could all this walking be bursting my leg's blood vessels and causing the bruises? Is that possible?

My left leg is bruise-free. The bruises don't hurt when I prod them.

Any suggestions? Thanks!