The Hammers shocked Tottenham 3-0 in a London derby prior to the international break.
"We proved against Spurs that we're always confident going into any game and that we can get a result anywhere," defender Tomkins told whufc.com.
"The Premier League is a funny league this season. There have been a lot of unexpected scores and on our day we can beat anybody.
"Swansea are good technically, but if we can play how we can, then we can get a result."
Tomkins returned to the starting line-up for the Capital One Cup win over Cardiff and has retained his place since, including in last weekend's loss to Manchester City.
"It can be frustrating at times when you're sitting on the bench, but I'm growing in confidence with this run of games that I'm getting," he added.
"I love playing for West Ham and hopefully I can help us bounce back at Swansea."