After the visitors managed to take a 19th-minute lead as Wilfried Bony continued his good form in front of goal, West Ham hit back with two fine Andy Carroll headers and Fabianski was sent off before Diafra Sakho wrapped up West Ham's win.
Fabianski saw red after referee Chris Foy ruled he impeded Sakho when running at goal, but Monk was unhappy with the decision, claiming the striker was guilty of an infringement first.
"It's obvious that he handles the ball to get past Lukasz, but I think the referee had already made his mind up and was adamant he was going to send him off," the Swansea manager told Sky Sports.
"Whether he saw the handball or not, you'll have to ask the referee.
"Whether it's an appeal or not, we'll have to sit down and have a think about that."
Monk felt Swansea fell below their usual standards as they slumped to a fourth defeat from six winless games on the road in the Premier League since beating Manchester United at Old Trafford on the opening day of the season.
"We had our chances but we weren't at the level that we have been and that's the first time this season that I can really say that," Monk added.
"Every other game we've played, we've very much competed and it's been close, but today was not at the level we have been. Some of the defensive mistakes cost us goals.
"Funnily enough, when we did go down to 10 men, I think we played our best football, but it shouldn't take that to make us switch on and play our better football.
"Unfortunately, we didn't deserve anything today. We didn't pick the right options with the ball and we didn't get on it enough."
