Trail NP poor grouping

I have had my Trail NP for a couple weeks now and it had been shooting great for me.* I recrowned the barrel and also done the bearing mod to the trigger as well as going over all the screws and adding loctite, but other than that it is stock.
While shooting last night one shot was really loud and pellet barely left the barrel. After that the POI* was about 6" low and slightly right at 20 meters. Before this happened I was able to keep the group around 3/4" at 20 meters. I have readjusted the sight to get back on target but the groups are much larger now.
I am new to air rifles and was just wondering where I should look first?

The really loud shot was the same shot where the pellet barely left the barrel, or these were two shots, one after the other?

In any event, the loud crack was likely detonation, and appears to be inconsistent shots now may mean the piston seal got some damage as a result/ If you have silicone lube, put a couple drops in the transfer port and see if things tighten up for a few shots- if they do, it is the piston seal.

The really loud shot and the shot where the pellet barely left the barrel was the same shot.
I guess I'll never know what the problem was, I have since returned it to the store where i purchased it.