You're not on the same page.
Hooking up with an ex is like walking into a burning house and expecting to leave without a burn, or at the very least, a serious case of smoke inhalation. It's just not going to happen. Chances are, you won't be on the same page. You'll have good intentions ("Sex and just sex!", you'll swear to one another), but things will inevitably get awkward and messy.
You won't be as over her as you think you are, or she won't be as over you as she'd like to be, and before you know it, you've fully unearthed all of the issues you took months to bury, and one or both of you end up hurt. Don't willingly get entangled in that web. Walk away—we repeat—walk away.