Well onion poisoning in cats (from personal experience because of a terrible accident) and dogs (possibly guinea pigs too, maybe?) goes into the lining of the GI tract, just like other nutrients, but from there is causes substancial red blood damage. It breaks apart the red blood cells, those are the cells that carry oxygen throughout the body. So severe anemia can set in and symptoms usualyl occur within the first 24 hours but can linger for days afterwards. Many symptoms include decreased appititte, reluctance to move around, weak, depressed, stomach pain, diareah, shortness of breath, rapid heart beat etc...
Usually if caught early enough fluids and oxygen are fairly helpful until the redblood cells can bounce back. Otherwise your pet will have to have a blood transfusion. Its fairly easy to tell on a blood slide if its this kind poisoning but not everyone knows what their pet ate.
If your guinea pig is acting at all like this I would get it to a vet. IF you or an animal loses approximately half your red blood cells (which is very dangerious) then it will take t least a YEAR for your body to fully recover. I'm sure that your GP didn't get that much damage or else you would know for sure.
Definately keep an eye on her. Give her a few more more iron rich foods, weigh her 1-2 times a day and maybe type/write out a list of all the dangerious foods for anyone caring for your piggies when you arn't there...