Hey there Narooma,
I am LBK since '92. Much of what you say strikes a chord with my own experience, but there are so many variables, that what I say might bear no relation to your situation;
1) Much will depend on your suspension mechanism and socket. I have the Iceross system.
2) Your prosthetist should have pretty good input on your problem, because, unless he/she is pretty new to the game, they will know your stump, and have seen this problem before.
3) My own stump started getting similar symptoms after a few months of walking - it was bruised flesh, not bone. If ignored, it eventually made a sore. Like you, if I rested it, the bruising went away until next I wore my leg all day. Because of huge variation of stump volume from morning to night, I would "sink down" in the socket as day wore on, and this would cause the bone on the front (shin) to locate just below the clearance that had been made for it by the prosthetist. This became worse the longer it had been since my last refit. At that time I was in a position to afford numerous refits, and had a refit of the socket every 5 months for the first 3 & 1/2 years post amputation.
My advice, therefore:
1) Check with your prosthetist, and see what he/she says.
2) Try adding a stump-sock; sometimes the added thickness can lift you a little - at least enough to avoid the sore spot.
3) Once you accept the need for a refit (sooner or later you will need it) ensure that your casting is done later in the day (when the stump has a lower volume) or ensure that you have a pretty tight bandage on for a couple of hours prior. This will ensure that the new socket will be as tight as can be - so you also need to make sure that you bind the stump prior to fitting and alignment to avoid delays in donning, and discomfort.
The first refit that I had after getting the Iceross system, I had driven two days to get the the prosthetist, and had left my leg off while driving. This caused the stump to fill up with body fluids and asume a larger size than when wearing the leg. The prosthetist cast me as I arrived and I had a fitting the next day. My stump had gone down overnight, and then I wore my (old) leg about for a couple of hours before going in to see him. He was amazed that the new socket was loose, just as loose as the old one! Luckily he had seen something like this before, and recast me immediately, while the stump was still "down". The following day I collected my new socket, and it was very tight. In fact I had to bind my stump either overnight, or for an hour or two before putting my leg on in the morning.
Not long after that refit, I was showing off my prowess at tennis, and rather overdid things. The next morning I could not get the leg on, and I was seriously depressed. I phoned the prosthetist, and he advised keeping the limb elevated, and applying ice (limited time of ice to max 15 mins/45 mins without) and after a couple of hours, I was able to get it on.
As I warned at the outset, this was my experience, and yours may be quite different.
As a follow-up, I must report that on subsequent refits, I have always ensured that I am cast after having worn my leg for at least a couple of hours, and no repeat of the problem. Also, as the years have passed, my stump has stabilsed somewhat, and re-fits are few and far between now. Also the fluctuation of stump volume has lessened considerably, but still noticeable - in fact, I am well overdue for a refit right now. Wearing two stumpsocks, and the socket still loose - I am scheduled for a refit in February, but in the meanwhile, if my leg gets too loose, I can take it off for an hour or two (I am mainly office-bound) and let the leg regain some volume before putting it back on again.
I hope that this little bit of info helps, and that some others jump in with their own experiences.