Could be one of a number of things,

like scabies

or have you changed any hygiene products lately?? or even washing powders etc.

Have you had any recent waxing or shaving (you don't have to answer these questions just to give you something to think about).

Do you have pets that live inside? fleas or some other insect.

Anyway hope this helps, you may have to go see your GP if it persists.

Goodluck with it, hope it goes away.