Playing with pets has suddenly become nearly impossible. For some reason, all of a sudden, my pets have started to reset themselves. This is usually when I let them do their own thing (which I do a lot), they're stood there idle until they attempt an action by themselves and then they reset, cancelling the action.

For example, if I leave them be and they decide to go and lie in their pet bed, they will reset and cancel the action, so they're basically stuck there constantly resetting. I don't have any mods for pets, so I have no idea why this is happening. Any ideas?

How do you know it "reset"? The pet jumps back to normal position, losing an action queue?

Depending on the size of the pets, some pets want to do certain things (that TS3 allows them to queue) and suddenly lose the queue because TS3 deems unable to do. I used to have a tiny pet dog (small size) that loves to play with something upstair but he cannot go upstair. He kept queuing for it and losing the queue. Unlike sims, pets don't have the tantrum with the bubble saying path is blocked.

But no matter what, I highly suggest you install (if not already have) NRAAS' ErrorTrap and Overwatch. These are very small mods (no performance hit, almost) and all they do are to isolate errors and not letting them ruin your game. Because if your pet indeed got stuck into a vicious cycle of script error, ErrorTrap would have stopped the script and send you a notification, and you can manually "resetsim" the pet, or, in most case, the game just goes on like nothing bad just happened.

Yeah, the pet jumps back to a normal position and loses the action. I've noticed this with all sorts of pets, cats, puppies, normal-sized dogs and it's when they want to do something as simple as sleep in their bed or play with a toy that's on the same floor as them, so I have no idea why this happens. I also have both of those mods.

do you remember any changes made to your game recently?
are you on the latest patch level? are your mod versions for the correct patch level? did you check for updates of nraas error trap and overwatch (newer bug-fixed versions maybe of the same patch level you are already using)?
also: did you check if your mods (hack-mods) could be incompatible? as far as i experienced the sims 3 - errors, glitches and mod incompatabilities dont need to be obvious / logic at all. meaning you dont need to have a pets related mod thats incompatible to end up with resetting pets...

did you already try to 50/50 remove some mods to check if its back to normal then? also dont forget to delete the scriptCache.package when you start testing (by removing mods) or update and rearrange mods