Thanks for your query.
If the stitches were non absorbable, then they need to be cut and taken out.
If you are 100 % sure that you found one after the midwife said that they had all been taken out, then there is a chance that one of them stayed there, or a part of it stayed after cutting the whole suture out.
Also, sometimes, after healing of the episiotomy
( your vaginal cut ), the subsequent fibrosis
and thickening ( induration ) feels like there is some prickly sensation, particularly on sitting.
In order to be absolutely certain that all of them are out, you need to visit your midwife/ treating obstetrician again, and have a thorough internal check up, including per speculum if required, and confirm that all stitches are indeed out.