Hey leozelig.
I have a tendency to do this at times...giving short replies to an otherwise long message. I don't know the reason why your penpal is doing this, but personally, I get busy sometimes, and rather than being rude and responding to the email a few days later, I write shorter emails. Maybe you could ask her how she's doing, how's school/work etc and see what her response is.
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Don't settle for someone who doesn't see your worth.