Stapelia variegata needs diagnosing!

[kiki-do]

Hi All!
I'm hoping someone can help me here. I have a Stapelia variegata that I've had a year and it's been thriving!
Recently, I noticed yellow/orange on three of the ends.
It's in a high light area with other cacti and has been all along. I water it thoroughly when it's dry. It's in cactus soil. I feed it cactus fertilizer once in a great while, maybe two or three times a year. Like I said, it's thrived up to this point. No signs of bugs that I can see on any of my cacti. Anyone know what is wrong? Thanks for any input.