My dog has had a lump beside his tail for 3 days now, it's squishy and feels like it is full of liquid, like a cyst or a human blister. he tends to sit down very hard on the wooden floor on that side so i presumed it was from that but thought it would go away quicker.... great form/appetite etc,  and no pain if i touch the lump.

Could be a heamatoma (spelling?) Basically like a bruise but it's where you get a broken blood vessel under the skin and so swelling without the usual bruise that goes with it.  It's been a while since I had one but I don't remember it being especially painful.  Treatment is largely the same as for a bruise