Almost everyone with a 4ish month old baby thinks their supply is dropping. The babies get much more easily distracted, feed more efficiently, and are generally having a few weeks of being grumpy too. Ride through it, keep offering, and you should be OK.
Try changing how you feed - in a quieter place? In a noisier place? Lying down on the floor? Give her something to hold while feeding?

Also - if she has a cold or an ear infection then this can cause feeding difficulties too.