is this normal or incest -not troll?

No, this was NOT incest and it certainly is not abnormal-- especially as a young child. The wind can blow a certain way and a boy (or a man) can get an erection. It just is a part of a male's normal physiology. Getting an erection felt really good, just as it is supposed to, and of course, not knowing any better, you wanted to feel that again. It does not mean you are/were perverted or attracted to your mother. Everything is in fine order, so please don't let it trouble you.

When you are 5 you don't associate an erection with sex. You associate being touched and getting an erection as oh that feels good. The same way it is normal to see babies touching themselves a bit it isn't anything sexual but rather they just know that it feels good. It wasn't incest, you just liked the good feeling that's all it's completely normal.

You liked the feeling of the erection not your mum.

Penis stimulation can occur at different times in your life for different reason. It could happen just becaus eit was any woman doing it, not particularly your mom. The mere fact you can remember this incident may be a bit strange, but no unheard of at such a young age.

Nothing happened so it's not incest (not that I care, for the record).

Are you just having a laugh? like, is this a joke? if it isnt, then NO its deffinatley NOT normal.