The 23-year-old was an ever-present in the Hammers defence last season, helping them achieve promotion back to the Premier League at the first time of asking.
"It is always hard of course," admitted Tomkins, speaking to London 24.
"I wanted to represent Great Britain and the manager gave me the chance to and I appreciated that.
"I went there expecting to play more than I did in the end, so obviously I wasted a lot of time and then Ginge [James Collins] and Reidy played a lot together in pre-season and the manager had his thoughts.
"It was disappointing for me, but I suppose that is what football is like. You have to keep your head down and keep working hard in training which I have.
"It paid off against QPR because I came on the pitch and did well."