That is a symptom that warrants you getting in touch with your team without much delay. Could be anything from a trapped nerve in your back, to the chemo.

I'm assuming it's your leg only? Not in your saddle area (like your buttocks, pelvis etc)? Your bowels and bladder habits are ok?

There's a condition called cauda equina syndrome that happens due to severely compressed nerves in your back and you need to rule that out as a matter of urgency. Doesn't sound like it is, but it can present like this.