I've been diagnosed with brain cancer and following the brain surgery my lymph node on the left side of my neck has been swollen and painful any idea?

Inflammation? Sorry for your diagnosis. Most primary brain tumors do not spread outside of the head. Since you've just had surgery it wouldn't be unusual for there to be some inflammatory changes or even infection which could cause lymph node enlargement. You should bring this to the attention of your doctor for possible treatment or further evaluation.
Challenge. May or may not be related to the surgery, but might investigate possibility of post-op infection. Of course, could be secondary to strep throat infection, or even dental abscess, and all of this must be sorted out. Primary brain cancer does not typically affect local lymph nodes, and thus, an alternative answer is needed.