They have active cycles and slow cycles, lasting from a day to a week(s). On the other hand, you're right to have a worry in the back of your mind: they do slow down when they're giving up the ghost and heading for death.
If nothing has changed about your water and you still do the same changing routine, the snail is doing what it does so don't blame yourself. One way to explore is to put food nearby (if it's at the top, put the sinking pellets under it). They can smell food and it might be motivated to go eat. You'll know it's fine if it's still going for the grub.
Hmm...wondering if it's lethargic because you fed a big meal. That would explain the extra poopines.
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