No convenia wonot cause side effects like that
But the side effects varies from pet to pet
To increase the appetite u can give syrup like aptiquick or revell 2 ml before food twice daily
The antibiotic will not usually cause these side effects, but it is definitely an indication that he is not improving with the current treatment. Please get a blood test done if you haven't already, get his temperature and other parameters checked. Most likely the attending doctor will have to treat him for mycoplasmosis with some other antibiotic like doxycycline/ enrofloxacin based on their clinical judgement.