What kind of chemo are you taking and how long have you been doing it? I ask because I have had similar issues in the past but they were not directly related to my BT or any treatments. My first thought is that you connect with both your oncologist and your primary physician to rule out if it could be:
1) The toxicity of the chemotherapy or a reaction to other medications you may be taking;
2) if you are not sufficiently hydrated and need to increase your fluid intake, electrolytes, and/or potassium (this is also a common occurrence in high-protein diets);
3) Consider getting checked for DVT (deep vein thrombosis), which I have found out earlier this year cancer patients are susceptible to, and to which brain tumor patients are ESPECIALLY susceptible. No one has been able to tell me exactly why this is the case, but I took a long flight to Europe earlier this year and had to seek medical care while there for what may have been a superficial DVT. It was then that I learned about the susceptibility of BT patients to these occurrences.
While I am not a medical professional, I offer these suggestions because of my own experiences with similar issues. I hope they serve as a starting point for dialogue with both your oncologist and PCP.