Last night I had arousing thoughts going through my head while I was sleeping. I would get an erection, and it would go soft after a little while. I would feel my urethra, and it was only a tiny bit sticky. Then, I checked under my trousers, and there was no mess whatsoever. Does this count as a relapse? Would I need to reset my counter?
No but obviously you wanna try and avoid anything that'll get you to that point because it could escalate next time
It kinda wasn't fantasizing. It was real images, and audiovisual thoughts of actual erotic content I've seen in the past. But is sleeping on my sides, or on my back, my best bet?
No don't try to avoid or fight the thoughts - this will make them stronger and put more focus on them. Even if you want to avoid them you still put focus on them and thus making them more important than they are. Just see them as thoughts and let them pass and put your focus on something else. A slippery slope is a saying - that if you do something now that makes it very easy to do the next thing as well and that a relapse is not far away.
So the thoughts just played in your head without any control from you? Don't sleep on your sides, the other way is better.