I just fed my pet 6 or 7 r99 food items it hadn't had before, and none of them are showing up in the achievement page. is there a specific page i need to go to for this one?

many tiered achievements are messed up. It can take time, say 24 hours before it updates. Some won't update until you actually hit the next tier. Some times going to the "collection page" - in your case, the foods eaten page, can force the achievement to update.

Check it today - its been several days since you asked about this. If its now updated, no problem. If its still not showing the added items, I suggest submitting a ticket.