What reason would a hospital have for keeping a baby for 4 days with these symptoms?

The baby's only symptoms were 102 degree fever and fussiness/not able to sleep. She is one month old. Also, they are running all sorts of tests with her--blood, urine (standard, I know) and they also did a spinal tap. Now, I would like to think that the hospital is just being very very thorough. HOWEVER--my cynical/realistic side is saying, there is something going on here. It's not like the hospital is going to get a bunch of insurance money--the mother (and baby) have a medical card. Anyone else think this is weird?