What could cause both canker sores and a yeast infection? could this be from a weak immune system?

Yes: Fungal ("yeast") infections are often secondary to other issues like a weakened immune system, as can be canker or other mouth sores. However, both can also be secondary to taking antibiotics, where the change to the normal bacterial components of your mouth allows fungi to thrive, resulting in both mouth sores and fungal infection. See a dentist and/or physician for exam and testing.

Usually stress: Followed by a compromised immune situation. Yeastn takes over when the normal bacteria are missing.