I've been biting the inside of my cheek since I was 6 years old, I'm now 26. Can this lead to problems?

Yes. Repetitive trauma to the mouth mucous membranes over time can lead to problems. In most cases, probably won't be harmful, but in the worst case could even lead to a cancer someday. Recommend you try to stop this.
Usually not. Aside from pain and discomfort, usually not. This might be caused by the alignment of your upper & lower jaws, or the teeth. Visiting a dentist might be helpful.