Shawcross said his side were well below par.
He told The Sentinel: "It was disappointing. We came here wanting to win and we've come away with a poor performance and no points.
"I thought the referee didn't give the free-kick and then the linesman did. But it is one of those things. You have to deal with the free-kick and we didn't.
"It's not been our greatest performance, we can't get away from that. It's something we need to improve on."