Technically you can give benadryl, this is a medication that we use in veterinary medicine. However, without knowing a history on your pet and having seen her in person, I would highly recommend waiting until morning unless you absolutely could not wait. Giving benadryl to a pet with underlying problems can sometimes hurt them worse than it helps them. For example pets with eye problems, heart, prostate and thyroid issues should use this medication only under direct supervision of a veterinarian that has decided this is the correct medication for use for the purpose it serves. This medication can also react with other medications that your pet may be taking. Above all, overdosing a pet (either because we don't have a correct weight on her or we are unable to be certain of how much to give) isn't good for her little body.
If you could wait until morning, this is what I would recommend. I would also speak with the vet that you see about underlying problems, if you have not already. We're coming up on the end of summer, most pets have already had the worst of allergy season before now and your pet's misery may be related to something other than just your standard allergic reaction to something that grows in her environment. If she is not on a monthly flea/tick preventative (just because you don't see them doesn't mean they aren't there), this would be my first recommendation. Pets with flea allergies usually itch in the back half of their body, around the base of their tail, legs, etc. She could also have an allergy to something she's around frequently (which I mentioned) as well as a food allergy. There are tests to check for most types of allergies.
Good luck with your baby, Rosie!
Sami