Just an FYI, they've changed the criteria in the new DSM so weight is no longer the defining feature and there's a sub-type of anorexia that includes binging and purging and there's also a pure purging disorder diagnosis.
Not that any of that matters; you obviously have an eating disorder and you're putting yourself in real danger every time you purge. If you can't stop, try pedialite as an electrolyte replacement. It's not great tasting, but it has the right ratio of sugar to electrolytes. They also make powder packets you can put in water and popsicles, whatever you can tolerate. You may also have low blood sugar after purging, so if you can manage eating something with simple carbs (fruit etc) in addition to pedialite, it may help. Dehydration is probably the main reason why you feel like crap though, you can also look up the World health organization's rehydration formula. It also has the right formula of water to electrolytes. You want to avoid plain water or Gatorade, powerade etc, they have hardly any electrolytes and you may just dilute your sodium, potassium, magnesium further. If you don't have anything else, a sports drink is better than nothing, but pedialite or its generic equivalent is better.
Sorry this is so long winded, but I had a really serious, permanently disabling neurological injury from abnormal electrolytes from purging, so I'm kind of paranoid about preventing it from happening in anyone I can...