Cat staying in one room, more or less in the same area?

For the past few weeks, my cat (about 13 years old, pure-bred Siamese) hasn't been moving out of this one room in the house. She more or less sits in either of two areas, one on a hard wood chair and the other on a lounge. But even when I take her out of the room, she goes right back.

What could this be? I haven't been able to take her to the vet; but there hasn't been anything, apart from her not wanting to move, that has really set off any alarms: it looks like she's been eating and drinking OK (possibly eating less, but whenever I put food in front of her she eats it), she's been sleeping roughly the same amount, she's not weak or limp when I pick her up, and she gets up to go to the toilet. Weight I'm not too sure about, but it doesn't look like she's lost or put on any weight.

If it helps, for the past several years, she's been vomiting very regularly, but the vet doesn't seem to think there's anything behind it (originally she thought it could be cat AIDS or leukaemia, but she must have ruled those out).

Thank you VERY much for your help!