Thanks for the reassurance, everyone! Cats are strange people, it's true and bossy people at that!
She's also been more vocal lately, using the sounds she makes when she's out of food or wants to go out even when she has food and doesn't go out when the door is opened. It's like she's trying to tell us something or warn us of something. Maybe those Mayans were onto something about 2012 and our animals are trying to tell us!
I'm going to be the lone voice of dissent here and say that if something feels "off" about your kitty, and she hasn't had a recent vet check-up (like, last few months) I would take her in for a visit. Only because you have mentioned that she is 13 years old - and while as kitties get older the most common problems to turn up usually involve fairly significant clinical signs, a common byproduct of the main problems (hyperthyroidism, chronic renal disease and even diabetes) is systemic hypertension which can cause strange vocalizations (and damage internal organs!).
So even if your vet just gives her a check-over, weighs her to make sure that she hasn't been gradually losing weight (one of the first signs of systemic disease in cats) and has a look in the back of her eyes (to check for retinal hemorrhages that come up as a result of high blood pressure) as a part of her general check up and everything turns up perfectly normal - you can rest easy that you aren't missing something.
It is entirely up to you of course, although I *am* a vet I am not YOUR vet and I have not seen your kitty so obviously take everything with a grain of salt.
I also tend to err on the side of caution, but that is probably because I see many, many older kitties that people have not brought to me because they thought "oh she's just getting older that's what kitties do" and it turns out they have chronic disease processes that haven't been addressed or diagnosed, and sometimes this has serious consequences.
If anything, keep a close eye on any changes to her drinking, urination etc. normal daily 'functions' including vomiting/diarrhea.
Anyway that's me off my soapbox!