Alexander Sørloth's late strike was ruled out due to a perceived foul in the build up and the decision was the cause of plenty of debate following the final whistle.
"If the game had gone on a bit longer we could have scored," Tomkins said.
"We were unlucky with the decision to disallow Alex's goal. You can look at it and say we were unlucky, but it's another defeat. We've taken the big teams close, right to the wire, but it's still disappointing because a loss is a loss, whether you win 5-0 or 1-0, we need points now.
"For me, not a chance it was a foul. I think Jairo pulls his leg away, even Gary Cahill got asked by one of the lads and he didn't know why is was disallowed. The referee's made that decision without a reaction from the Chelsea players, so it's disappointing, but we have to go again."
Aaron Wan-Bissaka impressed for the third consecutive game and Tomkins praised the young full-back's continued development.
"Aaron's done well since he's come in, to come into the Premier League and play against the teams that he has, that takes some doing," he said.