the other possibility is that those spikes are very heavy. While on her, they carry no weight because her power maybe transfers the mass somewhere, but once teleported, they become dangerous tools.

But that's a bit weak. I guess that if she has those spikes, she can do more than just have them use gravity, or that'd lead to a very limited use. I can't see the use of having those spikes if you're not able to throw them at incredible speeds in any direction. So I guess she knows a way using her power, or she'd use something else as weapons.