When we have persistent (or chronic) pain, over time our tolerance level (threshold) for pain can be lowered.
Activities that never bothered you before, or things like sleeping in an odd position, may alert an overly sensitive nervous system to a possible threat, thus cranking up the pain dial.
That long walk or a cricked neck is likely not any more of an issue than it used to be, it’s just that your pain threshold or tolerance level has been lowered.
You likely feel that response to pain quicker, more intensely, and longer.
This is not likely due to your body being weak, wrong, or broken. It’s likely due to nervous system hyper-sensitivity - your body's alarm system’s settings are on high alert. So stimulus that might have other wise not been a bother, becomes a quite bother.