In reply to RosiePogue:
This sounds distinctly like a heel bursitis to me. I've had exactly the same problem in the past, it's where your heel is swelling up with fluid to protect damaged tissue (probably) due to your boots impacting on the back of your achilles.
Basically, your shoes don't fit. Painful shoes are terrible, and you really don't need to downsize modern shoes hardly at all - just try lots of different brands until you find one that fits well. Personally I fit Evolve and Five Ten really well, and only have to go down 1/2 - 1 size at most.
Try and wear super soft shoes until the swelling goes down, ice/heat alternately and massage gently with ibuprofen gel. Then, when it has completely gone down, go buy some shoes that fit, and get rid of the old ones. Don't be tempted to start wearing them again afterwards or the swelling will just come right back.