So nearly 3 weeks later, most of the bruising is gone but I still have a rather big lump on my shin. Bit of a worry, might have to find time to go visit a doctor at some point?
I am no expert, but it must have been pretty bad if you still have bruising and a big lump. You are young and bruising itself should have started to fade fairly quickly, certainly within a fortnight.
My rule of thumb is if something is still there after a few weeks, I don't know what it is, it's still painful/looks different, then I usually bite the bullet and go and see a doc. Everyone has something going on anyway so a visit to the docs is never a waste of time.