If my baby has a lot of big, light brown spots on her body, should I be worried?

Maybe. If there are only one or two light brown (cafe au lait) spots, this can be normal. However, more than 5 such spots could be an indicator of a genetic disease called neurofibromatosis. These children with more than 5 spots and other symptoms like freckles in their armpits and eye findings need to be seen by a genetics specialist.
Ask your doctor. Light brown discoloration of the skin is very common. These may be "cafe au lait" birthmarks. These are usually normal, but depending on the size, number and distribution of the spots, your doctor may wish to do more testing. Multiple spots can sometimes be associated with an inherited condition called "neurofibromatosis". See your baby's doctor for more information.