A couple of possible scenarios:-
a) You are shooting with your sight set at the mark at which you were hitting 100 yards previously, and the rod has "suddenly" appeared. This sounds as if you are pushing too far with your bow hand palm, and hence twisting your bow upwards. Relax your bow hand, and concentrate on getting your bow arm consistent
b) You are shooting but gradually moving your sight lower because arrows are falling short. Was it a cold night in Harrogate (well it was in Ilkley!)? Were you tired, hungry? Had you warmed up? Were you really in the mood for practice? All these, and countless more, factors can affect your form, particularly early in your archery career. (probably sounding patronising here, not meant to be)
If this continues to happen, go back to a shorter distance (60/50 yards) where you could expect to have consistent groups. Watch what happens to your arrows there, and preferably get someone to watch you shooting. Look out for other things; are you pulling through the clicker faster, or with more difficulty; is your follow through bringing your hand to the same place, etc? In other words, get more data, and help in analysing it. And ask your club coach - it's what he's there for!