Most minor, single mouth ulcers are caused by damage to the mouth, for example by accidentally biting the inside of your cheek while eating, or from a sharp tooth, food or filling.
I have a 2005 Honda Element. The owners manual states the brake fluid should be replaced every 3 years. When I asked the dealer to replace the brake fluid, they said it was going to cost $190. I told them last time they did it, it was only $27.
You might wonder how often pads must be changed. It depends on how much menstrual blood you have, but it's a good idea to change pads at least every 3 or 4 hours even if you're not menstruating much. Naturally, if your period is heavy, you should change pads more often because they may get saturated more quickly.