After a session at the driving range I experienced pain in my right ribcage just under my chest.

I'm a right handed golfer.

Before anyone asks, I do not warm up properly.

Is this just mussel pain?

It does seem to get better and then worst when I play.

Should I take a break and for how long?

Try taking Week off,

Then when you do start to play again, WARM UP! especially in the Colder weather! It is very important to get the Muscles warm for any sporting event!

Why do you think the Pro's spend hours on the Range before a Comp? Its not just to practice! 

Jean,

If the pain goes away then it isn't anything serious.

Probably just some muscle damage and your not giving it time to heal.

I am not a great believer in warming up. A few quick stretches, a few gentle swings and that is enough for me.

Maybe you should just do chipping and putting for a few weeks.

Took a week off an my pain is gone !!!

I went to the driving range yesterday and streched before swinging.

I'm 100% today and hope for a nice roung of golf this weekend !

Thanks for the advice all.