Lowering blood pressure can have many causes. If it is consistently declining, rather than consistently low within a give range, it can be an indicator of an underlying condition. Definitely bring it to the attention of your physician. As chemo treatments go on, risks for infection and other side effects rise. So, I disagree with japdip. The effects of the chemo can be cumulative creating symptoms later in the process that weren't present in the beginning.
If I were you I would call this to the attention of your Oncology Team. I would think they would be taking your B/P when you are in the office for your treatments. Many factors can affect the B/P and it can change from hour to hour. That seems quite a long time since your treatment for it to be having much effect on your B/P now but ck. with the Dr. anyway. Regards ....