It sounds to me like it could be Rust, which is a fungal disease and can often attack plants that are suffering from stress due to the plant being in unsuitable conditions ie pot-bound or lake of water and nutrients.
First of all remove all the affected leaves and despose of them, burn them if you can.
Treat the remaining leaves to stop any further spread by spraying with a fungicide such as 'Dipthane' as instructed.
Re-pot if neccesary or otherwise remove some of the surface of the compost and top up to provide a thick mulch.
Mix in a pelleted slow release fertiliser with the fresh compost and keep it well watered. Good Luck. Tbird+