I can't account for the strange localized loose spot in the middle of the drum except to suppose that before the lanolin was put on, that part was already the most supple from natural grease from your hand and the lanolin just pushed it over the edge.
However, whatever the cause I suggest you put some warm water in your bath and immerse your drum head in it for an hour or two, agitating the surface of the skin occasionally to get rid of any treatment that might have over conditioned it. You must then remove it, towel it dry and allow it to dry naturally (not over a radiator). It may take a few days to dry completely and you will probably find that it is now too tight and you have to start again with any treatment (lanolin or hoof oil or whatever) but this method has rescued a couple of my drums in the past.
Alan Ellicott N Wales UK