My dog growls when I lift her.

Sounds like a dominance issue, but it could be the way you are picking her up, we really need more information here.

When you pick her up is she normally doing something? Playing with a toy, eating etc?

Does she growl at any other time like when you pet her or try to take something away from her?

How old is she?

For what reason are you lifting her and how do you do it?

She is a 5months old cocker spaniel I usually pick her up to take her from the room or put her in the car sometimes I lift her from her bed and she growles like a tiger. When I lift her I put my hand on her bottom and the other hand round her body. She doesn't growl if I touch her when she is eating or playing.

She is at a good age to nip this in the bud now being that she is only a puppy.

You will most likely get some more advice on this when the other pets experts come online but I will give you my opinion.

When you want to pick her up I would crouch down first and call her to you, you can use a treat to get her to come if you need to. Then once she is in front of you then pick her up (while still crouching) and hold her close to your body before standing up yourself.

It could be that you bending over her as seen as a threat as is suddenly being lifted. By crouching first and holding her while she is low to the ground she should feel more secure while she is being lifted. You could also try crouching with one knee out and getting her to jump up with her front paws on your knee to make it even less 'sudden'.

Growling is a warning and clearly there is something about being picked up that she doesn't like, if she doesn't growl any other time then I would say it is the sudden action that sets her off.