Ahhh its actually rather interesting you said that you experienced this after using for a prolonged period of time.
That gives weight to my theory that MXE has a metabolite with opioid activity, and seemingly with a long half life meaning that continued dosing would allow plasma concentrations (and effects) of the metabolite to continue rising over time, despite the MXE having largely worn off.
I'd say its this metabolite that would be responsible for your respiratory issues, as if it were to have a half life for example of ~8 hours where as MXE's would be more like 2 hours this active metabolite would keep building up, potentially with adverse effects (especially when combined with other CNS depressants). I among quite a few others notice a significant afterglow well into the next day which definitely feels like its from opioid activity, likely from the same metabolite.
People should keep this in mind for future dosing, as it looks like benders could be dangerous when dosing other downers during or recently after having an extended session.
Or I could be completely wrong.
Annnnd... that seroquel just kicked in.

To be resumed.......
Thanks heaps for the heads up fixed.
EDIT:: I cant find any pharmacokinetic info for MXE on google, doesn't look like we really know much about what happens to this chem after it gets the party going upstairs. Assuming it undergoes the same treatment as ketamine because of its structural similarities might prove to be risky when binging as hard as we all do on K

Anyone with more information would be awesome.