I've been diagnosis with a low grade, slow growing, brain stem glioma. about 3 weeks ago my left ear starting cracking and now my jaw hurts. is it growing?

Doubtful: How advanced is tumor? Have you had any treatment such as radiation to your head? This might cause fluid in the middle ear and crackling, perhaps some pain/pressure. The symptoms sound more like temporomandibular joint pain, which comes from clenching or grinding the teeth. It causes jaw pain, pressure in the ear, sometimes crackling sounds, & ringing in the ear to name a few symptoms. See md.

Impossible to tell : You will have to undergo a repeat MRI to tell, in general your symptoms do not fit with a enlarging brain tumor but i would discuss this with your treating physician who will then determine the need for a scan.