Cystitis is the most likely cause and is very common in cats; unlike in people, in which it is often caused by a bacterial infection, in cats it is usually a sterile inflammatory process, and nobody is 100% sure of why it occurs- it can be stress related, but not always. However, the incidence of bacterial infection does increase with age. There can appear to be quite an alarming amount of blood, but provided she is well in herself and passing urine well, I think she should be fine until tomorrow. However, it is really quite uncomfortable and she will feel much better after some anti-inflammatory medication, so it might be worth trying to get a morning appointment if you possibly can- although not necessarily easy after a bank holiday so don't beat yourself up if they can't squeeze you in sooner. As others say, increasing her water intake will help in the meantime- you could maybe leave a tap dripping as cats are often partial to running water, and, as odd as it sounds, putting a water bowl in a separate room to her food, and using a nice, wide, non plastic bowl makes a difference too. Also plenty of opportunity to pee helps- maybe an extra litter tray somewhere nice and secluded, where she will feel very safe, in addition to her normal one. Hope that helps and she feels better soon x