Obviously I agree that a vet check for a medical issue is first and foremost. The dramatic increase is a signal that something medically is off-- could be urinary track, hell could be crystal or thyroid or many things. A full evaluation is called for anytime your dog has a sudden change or increase in a behavior.
However, >>We've always had a bit of difficulty getting her entirely housebroken - she messes in the house every once in a while, but had a months long streak of being good.<< is not a great sign. Even if she has a medical issue, you still have a problem because you have a dog that is not housebroken and it is so much better to spend the next 3 or 4 months really addressing this than spend the next 8 to 10 yrs living with it.
Housebreaking is a bit like being pregnant... you are or you aren't. Sure, a sick dog doesn't count. But unless sick, or some crisis leaving a dog so long it has no option, pottying in the house is simply not okay.
So once you get the medical checked out, go back in time, and retrain as if a puppy. It's a little easier since she already knows to go out when you are home... but a whole LOT harder because when she does potty, you generally aren't. I see no way to do it without returning to crate training, not just when you are gone, but home also. Basically you are going to have to go out with her to potty, sing her praise, give her a treat. Take her out when she gets up in the morning, every 3 or 4 hours during the day, after meals, before bedtime. If she doesn't ever potty in the house at night, great. If she does, then her new sleeping zone is a crate. It helps if you have one where she generally sleeps and a 2nd on near the main action for during the day... because you need to start putting her in the crate when you cannot actively watch her even when you are HOME until you get 100 percent no house accidents. I'll put a couple of links with clear instructions.
Since this mostly is if left alone, separation anxiety might be the stimulus. Sometimes medications for a while, plus really good toys they only get when left alone (especially mind stimulating ones like Shirley suggested), Kongs with some smeared cream cheese or peanut butter (don't use a lot!! Keep a couple in the freezer!), safe chew toys, can help get the dog over the anxiety. Talk to your vet, because you can't effectively do housetraining while not helping with the separation issues.
http://www.vetstreet.com/dr-marty-becker/its-never-too-late-to-house-train-an-adult-dog-heres-how-to-start
http://www.canineprofessionals.com/housebreaking-adult-dogs
