The Royals looked to have stolen the points when Mathieu Manset scored in the 92nd minute - only for Craig Bellamy to lash in a free-kick from 30 yards some time later.
McDermott said: "I don't know where those seven minutes came from at the end of the game after the referee signalled that only four would be added on.
"Our goal came in the 92nd minute and it was quite bizarre that Cardiff's equaliser came in the 97th minute and I still don't know how the referee came up with all that added time.
"When we made it 2-1, I thought we had it won but then the referee came in with a massive decision to award a free-kick. I have seen it again and still can't understand why he gave that decision."
He added: "Every single game we approach, I do my homework and we have good players here with big hearts who do exactly what I ask of them. This is a tough place to come and Cardiff have great support but, in the end, it was a good point for us and now we have another massive game against QPR on Friday."
Source: PA
Source: PA