Sorry, I should have mentioned the dose. The dose of Benadryl is 1 mg/lb which is 16 mg. So, either 1/2 of a 25 mg tablet or a full tablet.
If you use liquid Benadryl at a concentration of 12.5 mg/5 ml, then 1 and 1/4th teaspoons would be her dose.
You probably know that sedation is a common side effect.
It's also possible that the wasp has nothing to do with Lizzie's symptoms and if that's the case, then I should have mentioned other possibilities.
I've seen dogs with anal gland issues behave as you describe so this might also be something to consider. I don't know if you're familiar with these glands, but this LINK
discusses them in detail.
Some dogs are extremely bothered when these glands fill up which causes pressure on their bodies and will continue to do so until they are emptied. Although a groomer can provide this service for you, they usually attempt to empty the glands externally. The better method is done rectally which is how a vet tech or your vet would empty them.