If I had to make a guess- it's gravity and the extra plasma that flows to that area during the healing process. When you're body is prone and parallel at night, your circulatory system moves things around easier. As soon as you get up, all that fluid moves south. Lay off the salt, (and alcohol or smoking if you do that) for awhile as those can affect water retention and circulation. Either way, it should clear up soon.
PS I am not a doctor or tattooer, but I've been injured quite a bit in my life ;)