The Newport Gwent Dragons forward was one of five uncapped players announced in interim head coach Rob Howley's 35-man party.
Coombs has been selected as a second-row after Alun Wyn Jones, Luke Charteris and Bradley Davies were sidelined for some or all of the Championship.
"It was a massive shock. I was just blown away to be honest with you," said Coombs.
"Obviously I knew there was a lot of injuries and I've been watching it closely, but I never quite believed I would get picked.
"I hadn't spoken to anyone in the Welsh management and that's the reason why I was so shocked.
"I probably thought they may give the heads up now and again. It just came out of the blue really.
"But it's happened, so I'm chuffed."
Coombs began the season as a back-row forward but converted to lock as a result of injuries at Rodney Parade.
The former Newport RFC flanker admits it has been a testing transition but it is one that has paid off.
"I got into the Dragons second row through other people being injured and it's exactly the same with Wales," added Coombs.
"I have played there for most of the season and I like playing there.
"It's something I'm not used to, but I'm just enjoying the role.
"The scrummaging is tough. It's a bit different in the back row. You just kind of hang on the edge of the scrum and get up and run.
"So it's a tough role, but I'm enjoying it.
"I'm abrasive and I like to get in people's faces. I just think you've got to give 100 per cent all the time. If you upset people on the way, you've just got to do it."
Coombs has made a rapid, and at 28-years-old albeit late, rise to the international arena.
But he is determined to make the most of the opportunity and will not be content until he pulls on the Three Feathers.
"Twenty-eight is probably quite late to get your first cap, but it's better than no cap," added Coombs.
"But I took the opportunity once with the Dragons, so why can't I take it again?
"I can't wait to get in there and I just want to get the shirt on and play. Obviously I've got to get picked, but hopefully that will happen.
"It's pointless just going in there and being part of it. I would just be wasting my time if I did that. I'm going in there for one thing and that's to play.
"I just keep trying and whatever comes my way I'll take it."