I can't recommend any specific treatment, as that would be practicing medicine on your pet without an exam, but I think you will find within the following link the information you are seeking:
Keep in mind, many skin infections, especially due to allergies, also have a concurrent yeast infection, requiring either medicated shampoos or additional oral antifungal medications like ketoconazole.
To differentiate what is and what is not on the skin, can be done by a simple skin cytology at the vet.
Customary is 22mg/kg every 8-12 hours.
Depends a bit on what is being treated. For example, a bad skin issue may be worth three times a day treatment, whereas, a urinary tract infection could get away with twice a day treatment, but for at least 3 weeks.
So, 80 pounds.
If it were me, I would probably have done the 1000mg every 12 hours also. Very very common dose.
Cephalexin is NOT designed as a once a day drug. Even if you look in that link, you'll notice that it does not say every 24 hours. Only every 8-12 hours. The medication requires a steady concentration within the bloodstream, and that cannot be achieved with once a day dosing.
If sick, you need to get in touch with your vet ASAP.
If, by chance, it has anything to do with stomach nausea from the antibiotic, giving with food can help, and you can give any of the following antacids to help settle the stomach.
To help settle the stomach you could try one of the following drugs. My favorite product is Pepcid (an over-the-counter antacid).
Pepcid comes in 10mg or 20mg tablets. You can give it every 12 hours.