I have problems walking in a straight line and with my balance in general. What could be causing this? Is it related to cluster headaches?

Imbalance. You need to get this checked, not sure if the diagnosis of cluster headaches is correct, you need to see your doctor and explain your symptoms. It can be from the brain or from an ear problem and the treatment differs.
Poor balance. Your symptoms are not ones usually associated with cluster headaches. You should consult your doctor who may choose to refer you to a neurologist for evaluation. If this has become a chronic problem, an MRI may be ordered among other tests to get an answer.