You didn't mention whether Mesh was placed for Umbilical hernia
If Mesh was used,there is more chance of getting infection as it is a foreign body.
There is chance of Infection even without mesh.
If the pus is coming freely,send it for Culture and Sensitivity and use an appropriate drug.
Till then use an Antibiotic with combination of Cefpodoxime200mg and Levofloxacin
250mg twice daily.
Dressing should be done daily till the oozing is subsided.
If you come back with Pus culture and Sensitivity report,i will further clarify.
Wish you speedy recovery.