I mean, it was made clear in KH1 that what changes the Keyblade's form is the Keychain at the bottom of it. I doubt she lost all her Keychain's at once. So what happened?

The Realm of Darkness is a special world so it could be that only Eraqus' keyblade was strong enough to withstand it's darkness.

Wasn't it that she sent her default keyblade along with Terra-Nort and her armor back up to the realm of light? So the only keyblade she had left was Eraqus's 

 

I know that doesn't exactly take account for the keychains but the above is what I always thought it implied :0

The Realm of Darkness is a special world so it could be that only Eraqus' keyblade was strong enough to withstand it's darkness.

Oh. I never thought of that. That could be true.

Wasn't it that she sent her default keyblade along with Terra-Nort and her armor back up to the realm of light? So the only keyblade she had left was Eraqus's 

 

I know that doesn't exactly take account for the keychains but the above is what I always thought it implied :0

this seems like the right answer

The Realm of Darkness is a special world so it could be that only Eraqus' keyblade was strong enough to withstand it's darkness.

 

 

Wasn't it that she sent her default keyblade along with Terra-Nort and her armor back up to the realm of light? So the only keyblade she had left was Eraqus's 

 

I know that doesn't exactly take account for the keychains but the above is what I always thought it implied :0

^ These ^

The story never takes keychains into account.  Unless it's plot-relevant, like with Riku and Roxas's keyblades, the game never gives attention or comment to a keyblade changing form.  Attention is never given to Sora, so we can assume his keychains are for gameplay purposes over story.  Aqua had given her keyblade to Terra, so she had to use Eraqus's.  In BBS, items used as keychains are eliminated altogether--new keyblades just sorta appear in your inventory after a new world or event, and that's it.

Wasn't it that she sent her default keyblade along with Terra-Nort and her armor back up to the realm of light? So the only keyblade she had left was Eraqus's 

 

I know that doesn't exactly take account for the keychains but the above is what I always thought it implied :0

My thoughts exactly.  Aqua used her Keyblade to save Terra and her armor, so it's likely she lost hers yet still had Eraqus's because she was sort of his protégé--in a sense.

Aqua gave up her armor and the Stormfall to save a friend. Since she used Eraqus's keyblade to lock the Land of Departure away, it became her own since it was still a live keyblade.

when she sacrifice herself to the realm of darkness is gives up her keyblade and armor to save Terra (and that explains why she never escapes)