ANSWER:

If this doubt came to you while you were in your Umrah, your saee is invalid and you must go back and complete your Umrah as you are still in the state of Ihram. 

But if you got this doubt after finishing your Umrah, your Umrah is valid.  

QUESTION: What should someone do If he does only 6 rounds between Safa and Marwa in his umrah. He noticed His mistake after cutting his hair