Hmm. Tetracycline belongs to the Tetracyclines group, of which Doxycycline is one, and that particular one is the drug of choice by one of my vets for treating guinea pigs with respiratory infections and was given to one of my pigs. None of the Tetracyclines should be used in pregnancy due to the risk of if affecting the bone development of the babies (I know this doesn't apply here), all in the group behave in similar ways and have similar warnings etc, though with small differences here and there.
Antibiotics in general can have an adverse affect on guinea pigs digestion, they are sensitive to the changes of the balance of bacteria within the guts and whilst this is potentially dangerous in anyone, guinea pigs are particularly at risk due to their guts needing to continually move in a healthy manner. Diarrhea is a common side effect of any antibiotics, and is particularly dangerous in guinea pigs. However treating with probiotics will counter this in most cases (to be taken a couple of hours after the antibiotics).
There are minor differences between the various Tetracyclines. Interestingly food can reduce the absorbtion of Tetracyclines, therefore reducing the dose that the body receives, the exception being Doxycycline, which is not affected by food. Being as guinea pigs must have food constantly going through their systems I wonder if Doxycycline may be better? Perhaps this is why my vet used this one instead of any others in the group.
If you are worried about any medication it may be worth contacting your vet and discussing your concerns.
I was intrigued to see if I could find anything specific to guinea pigs but can only find the general gut bacteria warnings, and this applies to all antibiotics in guinea pigs. Tetracycline taken long term can build up in the calcium crystals called Otoconia in the ear of guinea pigs, and these are involved in the stimulation of the little hairs that are responsible for transmitting sound within the ear, but I am not sure how hearing would be affected, if at all.
Here is some useful information about various antibiotics used for guinea pigs
Guinea Lynx :: Antibiotics
I'd like to point out that I am not a mine of information when it comes to medicines in guinea pigs, I tend to refer to other members who have more experience and knowledge of these things than I do, but I do pick things up here and there about drugs in general due to working in a pharmacy (currently low-level), but whilst we process vet prescriptions we are very much geared to human issues. As it happens I have been learning about this group of medicines at work very recently, but having looked up the Tetracyclines in guinea pigs specifically as I say I can't find anything to be overly concerned about other than the general gut bacteria issue and that perhaps Doxycycline may be better due to food not interfering with the absorbtion of that particular one.
Has anyone here had any problems with any of the Tetracyclines, or Tetracycline itself, with their piggies please?
I may be going over the top with this, but I wouldn't want you to delay giving the antibiotic to her unnecessarily.