Hello and welcome to hcm forum,
Cotton is placed above the access cavity to prevent food lodgement.
Please try to keep the cotton by yourself if possible,
The exposed tooth cavity can cause sensitivity.Its better to visit dentist
soon to get the treatment done.
Since you are under antibiotics,you can wait until the pus drains out.
Make sure that you maintain oral hygiene
Rinse your mouth after every meals to prevent plaque
and food debris accumulation.
Use soft bristled toothbrush and dental floss to clean your teeth.
Hope this helps.