Agree totally.

Spaniels are notorious for guarding objects, food, locations, in fact anything, and until you have built a good relationship with her, do NOT start with a ball.

If you want to start to train a spaniel to retrieve then you are better off back chaining it ie teaching the GIVE first, failure to do so is what creates conflict in dogs, competition and thus guarding.

Be warned.............

Joined a Cocker forum and ordered some books on Cocker training that they recommend

That should help aviod any issues

I would train the give command first, hand her the ball, then present a treat and trade her letting her know what a good girl she is, then give the ball back to her using the take command. When she is good at this then start throwing the ball a little ways from you so in her mind she will be thinking oh if I take this back to her I get a treat.

The stress may well be that she does not know what she should do with the ball and also if you have tried to grab at her to get it back. She may really want to play fetch but being unsure can cause a lot of stress.