Why would my scalp feel like there is fluid underneath it? it's all squishy over a large area.

Squishy Scalp?: There are certain limitations imposed by this format which include the lack of a photograph of the skin lesions about which you requested information, So it's impossible to tell you the correct diagnosis and/or therapy, My Best advice would be to call and make an appointment with the dermatologist. Your report is pretty odd because we don't usually see this type of problem so it's really necessary to visit the dermatologist.

Squishy scalp: Are there any other associated symptoms, like headache, or numbness/tingling in the scalp? Have you noticed clothes -like a hat- fitting tighter lately? There can be a number of endocrine as well as Dermatologic conditions giving you those symptoms, such as a thyroid condition called myxedema, or a pituitary problem, or even a rare scalp condition called lipedematous scalp. See your dermatologist