Panic not, unless all the leaves are suddenly very yellow.

If on each branch/twig, the oldest couple of leaves, those nearest the centre of the plant, are yellowing, that's normal for post-flowering every year, and will settle down in a few weeks.

If the leaves all over are looking just a bit less green, somewhat 'off', than they should, it's probably that the plant is hungry, so a good dose of ericaceous feed after flowering is a good idea. Liquid feed kicks in faster than granules, but granules will be longer lasting.

Sequestrene can help if the plant is in a sulk with its soil!

Of course, if it's none of the above, something has killed the roots and will need investigating...

If you need a bit of ericaceous feel let me know and I'll drop some off for you