Saints had arguably the better of the opening 79 minutes before the game burst into life when Hibs conceded a penalty kick, which substitute Nigel Hasselbaink saw saved by Ben Williams.
Lomas was sent to the stand by referee Craig Thomson soon afterwards and Paul Cairney added to his woes by netting what proved to be the winner after 82 minutes.
Midfielder Robertson was taken off on a stretcher and Saints captain Dave Mackay was shown a straight red card for an altercation with Leigh Griffiths in stoppage time as the hosts' misery was complete.
Lomas said: "It's been a disappointing night, especially with Davie Robertson getting that bad injury.
"Fingers crossed it's not as bad as we think.
"It looked like a block tackle and it appears he's come off second best. It's very disappointing for the lad and for us."
Lomas' dismissal from the touchline came after Hasselbaink took the spot-kick following David Wotherspoon's trip on Mackay.
"I kicked a water bottle," Lomas added.
"It was frustration. I said to the fourth official he could've used a bit of common-sense.
"It's frustrating, you've missed a penalty, not your regular penalty taker. I was a bit perplexed why he was taking it. Dave Mackay's normally our penalty taker."
Mackay was sent off late on after Griffiths chased down the ball which had been returned to Saints following Robertson's injury.
Lomas explained: "Dave suggested that Leigh came and put the head to him.
"It was a lock of heads and the ref just chose to send Dave off.
"If Dave has (motioned his head) it's stupid."