Sorry to know about your problem. The symptoms you are having can be explained due to many factors but the most important (you being a cancer patient) is metastasis to the brain. You should consult your oncologist
immediately and have yourself evaluated. He might refer your for an MRI/CT of the brain, which will help in picking up metastasis.
Once mets are ruled out, you can focus on other causes of such symptoms like orthostatic hypotension
, cervical spondylosis
Hope you get well soon.