Friendships are often based around being in the vicinity or direct contact with each other, so you have shared current events and common 'in the now' events to chat and swap ideas about.
It could be that without this mutual everyday contact your friend is struggling to make her emails seem interesting and feels that she may have covered much of what happened in the missing years already.
Then again, as the immediate excitement of your reconcilliation has worn off, she may just be a bit slower at replying. You can't keep up a fervent email swap constantly.
I would suggest waiting a while and when she replies, leave a gap before you respond, so that there is no pressure on either of you to keep up the pace!