Can you get brain eating amoeba from bath / shower water slashing in nostrils? (not forced up into nose) thank you

Not likely: There have been rare cases involving swimming in lakes or ponds. Again, this is extremely rare.

Where are you?: It depends entirely on how safe the water supply is. If the water is not from a safe source, and crawling with germs and critters from the water, you will get all sorts of bugs, including amoebae (and there are a lot of kinds of those, too) If it's from a safe source, you are likely fine.