I think he has nightmares about rattlesnakes? Sometimes he dreams of Alma specifically, other time he encounters snakes in a symbolic way - in the form of snakeskin or poisoned rodents, or vaguely undulating tangles of scaled bodies; but they're always, undubitably, a recurring omen. The worst is when he can't see Alma, but he can hear her rattle.
what do you imagine my muse has nightmares about?
just gonna take a second to scream ooc real quick because holy shit you read my headcanons??? i am going to kiss you on the mouth (or gently pat you on the head if you're not into that.)
anyways, you're right. lee frequently has nightmares about rattlesnakes, not just because of alma, but because of pierre mcconville's rattlesnake daemon as well.
for the first few weeks after killing his father, lee would have auditory hallucinations of a rattlesnake's rattle, especially at night or when he was alone (which went away after awhile but started up again after his confrontation with mcconville in the north.) hester would always reassure him it wasn't real, and that if it were, she'd be able to hear it, too, but nothing could quell his fears.
it began to escalate one night when lee awoke, but couldn't move. he could hear voices and sounds that had business being in the room he was renting -- his mother asking him, horrified, what he's done, his father telling calling him a murderer -- but the one that scared lee the most was the rattle of alma's tail. he opened his eyes, and could feel her crawling on him, up his leg, his torso, and finally peeking out of the blanket he'd been under. the whole time, he couldn't see hester and couldn't call for her, and he couldn't move. it was like he was totally alone with the snake, which shouldn't have been possible.
he squeezed his eyes shut, waiting for her to strike, but when he did, he suddenly became aware of a comforting weight on his chest, and hester's soft face pressing into his, trying to wake him up. the sounds stopped, and alma was gone.
lee wound up asking a doctor he'd befriended about it the next day, fearing he was going crazy, but they simply informed him what he'd experienced was sleep paralysis, and was advised to not open his eyes when it occurs and just wait it out until he could wake up.