Hello and Welcome to ‘Ask A Doctor’ service.
I have reviewed your query and here is my advice.
Long sitting can certainly be one of the reasons of swelling of the feet and ankles. But since has happened for the first time it is best to get a proper cause.
Hence I would advise you the following:
Get a consultation with your Doctor for clinical evaluation.
Rule out
anemia and hypo-proteinemia by blood tests.
Get the cardiac and
kidney status checked.
These will either confirm the cause or rule out.
And make possible any treatment if indicated.
Till then elevate the feet above heart level in lying down position.
Restrict common salt intake.
Hope I have answered your query.
Let me know if I can assist you further.