Paint bubbles - Whats going on?

Hi
Am just busy painting my house, and I have noticed that on some of the walls the paint is bubbling.
What am I doing wrong?
I washed the walls down before I painted them, just like it says on the paint bucket...
Thanks

You may have painted on a cold or wet day. This can cause bubbling. You will probably have to sand down the bubbles and repaint those spots. Make sure it is a clear, warm day to paint so you can open the windows to dry the walls.

Or could be the paint on the walls already and my need to use a primer.