Pictures here!

It makes no sense to me. lets see what the more learned say.

It looks to me that plant is not receiving enough light - new growth seems to be very etiolated. Has it been like that when you got it or since?

I am also wondering if there is drainage hole in the container, and how much/how often do you water.

I have read that if plant starts yellowing somewhere on the plant rather than natural corking that starts at the base, it is a problem that could be fungal or pests/bugs. I do not have experience similar to yours...wait for more comments.

Your cholla (Cylindropuntia) is overpotted and undersunned.