The Saints, who currently lie fifth in the SPL table, looked to have done enough with a goal each from Steven MacLean and Dave Mackay either side of John Baird's reply, but a late equaliser from Mark Stewart denied them the win.
And Lomas felt his side should have had the three points sewn up well before Stewart struck after appearing to be in an offside position.
"It's very much two points dropped. It can be a cruel game sometimes," Lomas said.
"We should have killed them off by scoring or through just being professional. Ultimately when you don't kill it off then the other team gets a chance.
"Dundee's goal was a little bit fortuitous but they got it and stuck it away so it's a missed opportunity.
"With the extra man you've got to kill the game off and we didn't do that. We've done it to other teams. There were some good saves from Rab Douglas but ultimately we should have been out of sight."