Some chemos make you light-headed, so if you stand up too fast you get a "head rush". Also if your platelets are low, you can feel faint. This could be what caused it. Keep an eye on things and if it happens again, mention it to your doctor.
It could also be a TIA. Here's what Google shows in an early listing ...
Transient Ischemic Attack (TIA)
A TIA is a "warning stroke" or "mini-stroke" that produces stroke-like symptoms but ... Unlike stroke, when a TIA is over, there's no injury to the brain
I would mention it to the doctor. Carcinista could easily be correct also.