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I'm afraid it sounds as if this little guinea pig may have suffered something we call an 'aspiration'- which means that the fluid entered the windpipe and went down into the lungs, instead of entering the oesophagus(food pipe) and going into the stomach.
The way you describe her mouth open wide sounds as if she was gasping for breath, which the fluid in her airway may have been preventing.
Guinea pigs cannot swallow large quantities of liquid quickly, and so it is possible for fluid to enter the lungs if it is instilled too quickly.
I hope this is helpful
Best wishes Andrea
Ideally it should be given in small increments, as drops rather than squirted in as one dose.
Its unlikely that anything you could have done would have helped- occasionally giving mouth to mouth (or oxygen if at the vets) can help-but in most cases these little guys die of a combination of shock and aspiration of fluid. I'm so sorry that you lost your little guinea pig. Andrea