Can she hold her head up at all?

If she's on her tummy, does she lift her head to look around. At that age i'd expect a baby to be at least able to lift her head, even if she can't hold it long.

Put her on her back, and pull her gently towards a sitting position by her hands- does she bring her head up towards you with her shoulders, or does it just hang back ? She sould be bringing it forward at least a bit by now.
If her neck is really floppy still, or it is worrying you - get her checked out, or even just mention it at her next immunizations.

Maybe you've been over carefull about always supporting her head, or she hates being on her tummy- and she hasn't had alot of opportunity to build up some muscles?
If she doesn't seem especiallly floppy in ger=neral- like she has controll of her little arms and legs, then I wouldn't think it could be anything too serious.

I have no idea on this one..Because my son seriously could hold his head up the day he was born...It took him about a week and a half out of the hospital to have control where he didnt wiggle around and pop it back and forth...But all babies are different but if you are worried then ask your peditrician

It's different for different babies. Just make sure you give her enough tummy time. One of my nephew could hold his head up at 3 1/2 mos. Another couldn't hold his head up until 6 mos.