You may be causing more damage by giving penicillin injections so first you want to stop that. Even though you likely didn't give enough to do anything, giving low amounts of any antibiotic just makes it that much harder to fight it when the correct antibiotic is chosen, as some bacteria have learned to be resistant.
Pneumonia doesn't just pop up out of the blue- it is often secondary to other coughing/lung issues/viruses. Also there are a wide variety of things that her symptoms can signify, just some of them I'll list here:
heart disease, aspiration, pain (from anywhere in the body), heartworm, lungworm, fluid build up in the chest, enlarged organs in the abdomen (which put pressure on the diaphram), blood pressure irregularities, kennel cough, upper respiratory infections, sinus infections, asthma, etc etc etc.
Unfortunately you're not going to know what is going on without a vet visit, but since you are concerned and labored breathing is a severe symptom, you do want a vet to diagnose this asap.
If you have financial concerns, you can apply for care credit at the vet's office:
http://www.carecredit.com
Best wishes,
Christine
Certified Veterinary Technician
Licensed veterinary technician (B.S. Mercy College), 10 yrs in animal medicine and training