In Robert Kirkman's Invincible universe, the characters Robot (Rex) and Monster Girl (Amanda) go through a portal to the Flaxan's dimension during a battle (in issue 71, if I recall correctly). Before entering the portal they were extremely close - they were around the same age, Robot helped keep Monster Girl from aging (creating a belt and a refined version of Black Samson's suit), and were just very close emotionally.
Sometime later (around issue 81-82, I think?) they return, having aged significantly. At this point they are emotionally distant, and Robot is very hurt by actions Monster Girl took in the Flaxan dimension.
What happened to cause the rift between them, and in what issue of what series did it occur?