Always says the wrong thing

My girlfriend left her cell-phone charger here in my dorm room on Monday, when she left. She's two hours away, and I can't necessarily drive out to see her.
I should learn to say things in a positive aspect when the time calls for it. Like last night, here's the conversation:

Her: Call me tomorrow?
Me: *Remembers her cell-phone is running on limited battery. Searches a way to say this, finds it very hard* I'd call tomorrow, but your cell phone is going to die *I wait for her to offer to call me from her home phone*
Her: *nothing*
Me: *oh shit* Um, how are you feeling right now? I didn't mean it like that
Her: Why do you want to know?
Me: Because I care
Her: Here's what you just said "Call me tomorrow?" "No"
Me: Damn, I can see how that looks bad. *I didn't mean it like that at all*

-Should have said "Sure, but remember your battery is running on low"