I'm sorry to hear you are having this issue with Tiggy, which must be a little unsettling.
It is very true that some cats are home bodies while others use our homes as hotels where they eat and sleep and see the outside world as where they belong. However, I appreciate in your case that it is more the change in behaviour that is causing concern rather than the behaviour itself.
As she is now a senior lady, the first thing I would want to do would be to ensure she has had a recent health check. At her age, we can start to see the development of many health issues that can cause altered behaviour. Often, these conditions are tricky for owners to pick up on. This can include sore teeth (periodontal disease), the beginning of osteoarthritis (joint disease), cognitive disorders (such as feline dementia), hormonal conditions (such as hyperthyroidism, which can cause increased appetite) and renal disease. A vet can assess Tiggy and give her a nose to tail check. At her age, it may also be worth running some simple blood and urine work and checking her blood pressure to ensure it is not raised.
Given that the running can be associate with food, I would be especially keen to ensure all is ok in her mouth; checking for any gingivitis, resorptive lesions, decayed teeth, ulcers, masses etc.
You mention you may struggle to make an appointment for her, so would perhaps benefit from a vet clinic near you that does walk-in appointments? Alternatively, schedule the appointment for after a meal time and ensure doors are kept closed when she is eating!
While my priority would be to check for health issues, there are other things to consider.
Have there been any changes within the home that make her less likely to want to spend time there? These may not always be obvious. Things to consider would be any new pets, new people, different smells (cooking fumes, air fresheners etc), loud noises (people shouting, tv or radio on full blast), lack of personal space (children trying to stroke her or pick her up), having to share territory (with either people or animals) etc.
To try to encourage her spending more time indoors, there are a few things we can try.
First, I would not advise forcing her to stay inside as this will likely result in stress and behavioural issues and can even cause health issues such as cystitis.
What we need to do is to make the home environment the perfect safe space for a cat so she chooses to spend more time there.
As well as having her own bed, bowls, litter tray etc. she would ideally have her own cat tree, toys, interactive games, puzzle feeders etc.
She needs a designated zone where she is not bothered by anyone that she knows she can go to and relax without being looked at/stroked/picked up. This may be her bed in a quiet room in the house.
I would encourage the use of a Feliway adaptor within the home to release calming pheromones.
Ensure she is not being fed elsewhere and consider a collar that says 'do not feed me' if you are suspicious of the neighbours. Be sure to not feed her outside of the home.
With the recent warm weather; consider if inside is too hot for her, or if you have A/C or fans, if it may be too cold.
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