Well, I doubt Lena’s power is inexhaustible, and she may have been worried that they’d need it more later. Or, if she knew the sword was magical, she might have been afraid how her magic would affect/be affected by the blade.

Wow how did Lena stop time even know she was still in her pajamas

@Brandenr11 um...because Lena's powers are in no way connected to what she's wearing or carrying? Unlike Magica she doesn't need a magic amulet, or at least she didn't by this point in the series, to use magic.

How did Lena just frozen time anyway I had never one before doctor strange used to had the time stone but he gives to thanos