Lump on groin

[Vercingetorixraktavija]

My son 11 has a huge lump on his groin area. It's very puffed up on one side. First I thought one of his lymph nodes swelled up, he was coughing and had general cold symptoms. But today he has shown me the area because it was too painful. I have asked him if his can feel both of testicles since it's another thing I can think of. He says both his testicles are down.
But one side of his groins are really puffed up and really painful to touch he says.
What can it be? It doesn't seems as urgent as we need to go to A&E, so we are going to see the GP in the new year when GP surgery is open. He is eating well, and not too sick, except for the pain.

Thank you everyone. In a way, it's you great people who made me seek advice sooner and helped my son from suffering irreversible damage.

@FlissMumsnet (MNHQ) Thanks, nice to know that MNHQ really cares for members.

One thing I want to mention is, that as BabyST mentioned, he has indeed suffered from kick in the area in the past, during participating in contact sports. He seemed fine afterwards. We really don't know the cause of this, but it could be. It was about 6 months ago, and he was fine so we never seeked medical advice after this. And too young to know something was wrong to tell us earlier. So, even though the child seems fine, do check it out if they suffer any damage to the area.