Can you post a video of her breathing? - post it to youtube and then put the link on here
What do her lungs sound like when you hold her up to your ear and listen to both sides of her chest
Is Benadryl eating and drinking?
Any weight changes?
Does Benadryl hang her head down or hold it up to try to breath easier?
Are her sides working hard to breath?
It sounds like an infection or pneumonia
http://ratguide.com/health/lower_respiratory/pneumonia.php
Benadryl needs to see a vet asap and likely needs to be immediately put on antibiotics (orally, twice a day for at least 4 to 6 weeks), and other medications (possibly dexamethasone and a bronchodilator - see link above)
It could also be a heart problem in which case she would need heart meds - see links below
Here is some information:
http://ratguide.com/health/cardiovascular/cardiomyopathy.php
http://ratguide.com/health/lower_respiratory/pneumonia.php
http://ratguide.com/health/bacteria/mycoplasma_mycoplasmosis.php
http://ratguide.com/health/cardiovascular/congestive_heart_failure.php
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If Benadryl needs to be put to sleep by a vet to prevent sufferring - most vets do not know how to do it properly so if this is done, then you will need to make sure it is done right. The rat is given the gas/oxygen mix used for surgery. When deeply unconscious her eye and toe reflexes are checked. When unconscious to at least a surgical level, the vet will inject her. She will need to stay unconscious, breathing in the gas/oxygen mix until she is dead. Vet will check for a heart beat twice, several minutes apart.
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I am very sorry this is happening.
If it is an infection or myco, a combination of antibiotics will hopefully heal her
(antibiotics are given orally. twice a day for at least 4 to 6 weeks if they are helping. If they are nor starting to help after a few days then the vet needs to add another antibiotic or prescribe different antibiotics)
If it is a heart problem then medications may give her more good quality time, depending on the heart issue and the progression of her heart disease
I am messaging lilspaz68 as she knows much more then I do
Another place you can post in the facebook group Real Rat Lovers Want To Know