There should never be pain, ever. If your muscles are tired or burn or it just hurts in this general sense that's normal BUT that kind of pain...never.
Don't do what hurts, period. It's not worth it. When you can 100% say "It doesn't hurt" you're ready for it, until then you're not.
But it's just very important to re-check your form. Are you in neutral spine on the bench or floor?
http://robertsontrainingsystems.com/blog/coaching-neutral-spine/Are your shoulder blade back and down?
Are your elbows just slightly bent?
Do you stop around arms parallel to body?
If your form is solid and it still hurts, then don't do it and start stretching the sternal head of your pec major.
http://www.exrx.net/Stretches/ChestGeneral/Doorway.htmlThen check the movement again after a few days of doing the stretch every day for 90 seconds a day to see if you feel any pain.