Between 1.15pm and 2.15pm on Saturday, November 24, the woman was in Dunkirk Avenue when she noticed a blue/grey car which seemed to be following her.
When she looked, the male driver was exposing himself; he then drove off.
The man is described as white, with a black moustache.
He was wearing a blue hooded top which covered most of his face.
Witnesses, or anyone with information, should call Northamptonshire Police on 101.
Alternatively, they can contact Crimestoppers anonymously on 0800 555111.