Phal leaf yellowing do I need to worry?

[sweta]

Hello friends,

I bought this phal from a retail store about a year ago. I repotted her after it finished blooming last year and since then its grown 2 new leaves and lots of roots. Its got a very strong root system. It skipped blooming this year for some reason. Recently I've noticed one of the leaves that is not the bottom one has started yellowing and I'm worried whats wrong with it. I dont see any stem rot. Is this something to worry about? I've quarantined the plant and I'm hoping there's nothing wrong with it. In one of the pictures I've highlighted in yellow the leaf thats yellowing. Would appreciate any advice.