Dog has lump on top of head.

My great dane will be 1 year old on Dec.10th, 2010. We had gotten him from a breeder when he was 3 months old. He has never really had any physical/medical problems that we know of. When I woke up on Sat. (almost a week ago now) morning and went to take him outside I noticed that he had a large lump (about the size of a baseball) right on top of his head between his ears. I thought maybe he had hit his head on something, but my husband and I could not remember him hitting his head at all the night before. It just appeared over night. Also, it is not a hard lump it's almost like it's filled with jelly, but now the core of it seems like it might be getting harder. It doesn't seem to bother him at all or be painful. We figured that it would go away or at least start to get smaller after a few days but it hasn't seemed to get any better. We can't really afford to take him into the vet right now if we don't have to, so is there anything we can do or do you think it will go away on it's own? What do you think this lump could be?