I hope I can express this properly.  I have a 14 wk king charles and today after having him for 4 weeks he ws left in alone for 3 hours I came home for 1.5 hours and then he was left for another 2 hours in his cage

Anyway tonight I n oticed he was a bit and only a bit agressive just growling a small bit and yelping when i lifted hm down.  I touched his stomach area and it seemed ok however there is a bit behind his "twinkle" and his backside that he wont let me near.  Now forgive my lack of knowlege there is a bit that has more hair round it as if it is ment to be there but can't think what used for.  as I say he wont let me near it.  I had him away for day yesterday and we were walking in fields..could he have maybe picked up something or is it just a case of him biting himeself today.

please help I am so concerned

I'm afraid i'm stumped. If he's yelping every time you pick him up and acting out of character I would pop him to the vet to be on the safe side.
You say the bit behind his twinkle. Is the bit by his testicles or further forward. At his age he could be very sensitive around his genetils. Have his testicles dropped or started to drop yet? If he has any lumps around that area apart from his bits I would go and get it checked out.