Here is a guarantee: at some point, and probably several :lol:, no matter how well housetrained a dog is, it will have accidents for no discernible reason. One of the most popular places to have such an accident is on the owner's bed. Beds are full of scents the dog enjoys and it may merely be adding its own. Lily did this on my bed a couple of weeks ago and she has been almost perfect in the house ever since I got her. In this particular case with Chloe, while heats can cause all sorts of behaviour changes, keep in mind as well that 8 month isn't really old enough for a puppy to be fully housetrained -- it would be extremely rare not to be having occasionally accidents through to age one.
Often -- if the dog is already allowed the run of the house and can spend time out of your sight -- I hate to say this but the likelihood is very high that this isn't a one-off but that she may well be going inside now and again when you don't notice. Cavalier wees can be extremely small and unnoticeable on a carpet or rug or even on a floor that isn't being checked (especially dark colours! I only recently realised one of my cats has been weeing on the small, dark coloured bathroom rug for ages -- they dry quickly too and I never noticed any dampness but clearly he was doing this
. I couldn't
believe I hadn't noticed because he has gone on the floor as well since I removed the rug -- that room now has to be completely closed to him while I try to modify that unwanted behaviour! -- and the volume on the floor when he did go was as much as one of the dogs! So, it is indeed very easy to miss what you think would be pretty darn hard to miss!).
Overall: I would not allow a dog under 12 months ever to have free rein in a house and be out of sight in rooms that I am not in and watching, especially any rooms with rugs, carpets or soft furnishings like beds or sofas. I have learned from my own frustrating experience that this is never a good idea! If they wee into a mattress or sofa it will be well nigh impossible ever to remove that scent, for them to return to and pee again. And that we give the dogs far more credit for being further along in a training process than they often are, usually because we start to slack off on training on the assumption that they've 'got it'. When you think how quickly they can slide on basic obedience commands that aren't constantly refreshed and positively enforced, it's amazing they don't have more housetraining issues than they do.
I'd be very cautious at keeping her always in sight and in reach and be sure to continue to positively reinforce with opraise and occasional treats, the right housetraining behaviour. I'd be very cautious about giving her access at all to the bedroom unless it is at the point where you are going to sleep, if she sleeps with you.