Trust a crisis to happen in the middle of a weekend! I've been there many a time and feel for you! Good that you have been able to speak to a vet. As long as your girl is still eating and not in any major discomfort, it is safe to wait until tomorrow, as long as any sheer bleeding is just drops and not gushing.
If it is urine mixed with blood and not sheer blood, then it is likely to be a UTI. It can sometimes start as a drip of quite dark red colour, but should develop into more frequent and rather smelly peeing.
Bleeding can happen if a small stone is being passed; in this case, your vet is likely to not find much if a small stone has actually been passed, but the bleeding should stop fairly quickly in this case. If a larger stone is stuck somewhere in the urinary tract, it can lead to bleeding. If is stuck just before the exit you should be able to feel it. It is possible for a cystitis/infection to develop in the wake of it.
However, there is always the possibility that something has gone wrong with the womb in sows; that is a possibility that should never be discarded, especially with an older sow.
Bleeding from the gut, especially if your piggy is still eating, in no visible discomfort and the digestion is normal is likely not so much an issue in your case.
Unfortunately, it is up to your vet and a scan/x-ray to see what is really going on, ideally one with no as little of a whiff of gas as possible with an older piggy. Anything else is just guesswork! At least it is good that your vet is aware of various options and not just scooting for the for most obvious solution.
Make sure that your girl has plenty of fluid and is comfy and warm (but not hot); change her cosies regularly. Weigh her daily at the same time to monitor her actual food intake; it can be deceptive. if you are given any antibiotic, please also give a pinch of probiotic about 1-2 hours after the antibiotic to bolster the guts. You can ask your vet whether he has some. We recommend avipro plus if you are dealing with a bladder issue, but any will do in a pinch. Please ask for a painkiller if she is in any discomfort.
I wish you all the best. Please keep us updated!