Presuming that you have been diagnosed as a case of Pneumonia, based on X-ray of the chest and blood tests, and are being given antibiotics for resolving the suspected bacterial infection.
Per se, if you are on orally administered antibiotics, they are generally considered compatible with flying duties. Important issue is 'the infection' you are being treated for, as well as, chances of spreading it to others.
Each group of antibiotics has its own side-effects, so also you may be on other medications for relief of pneumonia associated symptoms. So, the disease and the treatment needs to be considered for your being safe in flight, in turn flying safely (assuming you are a pilot).
You may like to PM me in case you have any clarifications to seek.