I know it seems dumb, but my logical mind is saying this sentence I just popped out is all wrong. This is why: My mostly non-fiction writers mind is saying that the sentence implies the mattress itself is under the blanket and not the narrator. I'm just wondering if I'm overreacting is all.
I think it's clear in that sentence but I often encounter this problem because my brain processes things this way. I just solve it by switching it up if there is any doubt. I know there's some grammar rule like split infinitives or something, but I'd have to look it up to know what it's called. I’m lying under a scratchy blanket on a firm mattress. It's easier to change it than worry about it. If you worry too much you'll worry about being under both the blanket and the mattress the blanket is under.
I like this muchly. There is a counterposing in this construction that is more satisfying than in the other.