Cat suddenly peeing in house

[Tigger2019]

Our 5 year only girl just started peeing inside about 2 weeks ago. Prior to this never had any accidents at all inside. Always used cat flap. The only change is Christmas decorations up about a week before it started. House is open plan lounge and dining room. Christmas tree -artificial - is in lounge. So far she is only going in that room. Lots of meowing at same time. Called vet last week who said it's likely to be stress from the Christmas decs. In the meantime I noticed blood in her pee and she's only doing small amounts. Took her to vets who diagnosed cystitis and prescribed anti inflammatory and cystaid. Trying to keep her out of the lounge but she seems to make a bee line for it. The vet seems to think she'll stop when the decorations are taken down. I can keep her in kitchen with cat flap at night and she's not done any in there. So far none upstairs as far as I know. It's strange because she's seen the same decorations for 5 years and not had any issues. We can't think of any other change which could be causing her stress.
Just wondered if anyone else had any similar experience and any advice.

May I suggest taking the decorations down and also trying some Feliway plugins?

Thank you tortiecat, they're coming down tomorrow so hopefully that will help her. Have you tried Feliway could I ask?

Try an additional new litter tray maybe?

Try feliway optimum, it was the answer to our cats problem. Normal feliway made him worse.

Happened to us and she had a urine infection. After it cleared up she stopped. She normally goes outside but she started doing it in front of us in the kitchen, probably trying to tell us stupid humans that she wasn’t well. Pretty clever way of communicating even if it took us a couple of days to realise that she wasn’t just being naughty.

Pet remedy is also good OP. Animed and Viovet sell it.

Thanks for all your replies. Decs down tomorrow and Feliway ordered. Fingers crossed she'll feel better.

Go to the vet to check for a uti. Our boy did this and we saved him just in time (but bladder blockages are more a thing in boy cats).

That happened to one of mine, including blood in pee. 6 months before it all started my other cat was having different issues and the vet told me to put him on dry food only. He was a little bugger for eating his and her food, which meant he would eat wet food. So I gave both dry. Turn out some female cats can't handle just dry food diet. It might not be in your case though x

Vets checked urine sample and says no infection/UTI. Thinks it's definitely cystitis.

Cystitis treatment completed, no blood present in urine. Decs down since 29th December. Feliway optimum diffuser plugged in. Spray also used in corner of lounge she has chosen to pee in. Area cleaned really well. She just came downstairs while I was working and didn't see her, went to the same corner and peed, then went out the cat flap!!! She's clean around the rest of the house as far as I know- haven't seen any evidence of her going anywhere else. I'm getting a bit overwhelmed with it now, I'm hoping someone else has got some advice.
Anyone got any other ideas?

Have you used an enzyme cleaner for the area she has peed? If not, this may help.
Does she have a litter tray or does she normally pee and poo outside? If she has one and the treatment with the enzyme cleaner does not work, you could put the litter tray on the spot where she has peed.

Thanks @Mia184 I have tried that yet. I will try to get some. It's laminate floor and I've used white vinegar, zoflora and other pet urine sprays, but they're not enzyme so I think that's the next option. She doesn't have a tray she normally goes outside, which she still does. I'm think she's just got into the habit of doing one in the lounge as well, possibly because she can smell it (even though we can't).