cracked or damaged nozzle...replace the head system
clogged nozzle....clean out the nozzle with a toothpick
paint too thick...thin your paint
From your description of the problem I would begin by cleaning the nozzle.
If the pattern is still distorted replace the head system or at least the fluid nozzle.
There are some good tips on maintaining paint and reducing the possibility of contaminates getting into your brush in this post: tips and tricks