Generally the most narcotic gas we regularly encounter in diving is Carbon Dioxide which is about 20 times more narcotic than Nitrogen. Any kind of work that can lead to increased CO2 can also result in a big increase in narcosis, this can make deep narcosis a lot worse but can also bring on less severe narcosis at shallower depths. Add cold temperatures and bad visibility into this and narcosis at 20m/70ft is possible, although it shouldn't be too bad!
One of the main symptoms of narcosis is a lack of awareness what is happening around you, this is normally easier for others to spot than yourself so it's quite hard to really know just how much you were narked. The main thing is to appreciate everyone gets narked, we normally deal with it but if a situation suddenly gets bad and increased breathing, worry and CO2 build-up sets in it a lot, lot harder to solve than it would be with a clear head and no narcosis.