Obviously if the dog is IN PAIN the dog needs treatment esp if it is refusing to eat, drink, loosing weight move and is lethargic or in your words..."lazy" versus how it used to be; chubby, happy & active.
FYI: it is a FELONY to withhold veterinary treatment to a SICK or injured animal, even if you LACK the funds to pay for it. You can take it to, or surrender it to the SPCA for treatment. The limping leg may be a torn ACL or slipped patella. The refusal to eat may signal broken and/or infected or abscessed teeth. (The weight loss is from not eating, or from cancer.) the dog could have developed fluid around the heart (congestive heart failure) or other offshoot problems, from the pneumonia. We cannot SEE or examine your dog & we cannot do a MIND MELD with it, so we have NO IDEA what is wrong. I strongly suggest you PAWN something or borrow the money to go back to the vet. "Credit Care" might be a good option to look into.
http://www.credit-land.com/medical-loans.php