Pain is usually your body's way of telling you that you are doing something injurious to yourself. You'd do well to listen to it.
Additionally, most people make the mistake of thinking that since they've been lifting for a while they can just jump into any exercise with loads that are too heavy, until their body has had time to adjust to the movement.
Start light, it is probably even better to do the same weight for a specified number of sets for a couple of weeks, work on form, and add (a little) weight onto the last set once you've acclimated to the movement, then you can start progressively increasing the weight....
You may also want to try cable flyes instead (assuming you're talking about DB flyes), which are a lot easier to customize, regarding form, angle, ROM, etc.
And I prefer these to DB flyes b/c the muscle is always under tension, and the contraction is more intense at the completion of the movement. In fact, I've replaced flat benches with cable flyes, since the primary function of the pectoralis major (sternal and clavicular head) is tranverse flexion with the should internally rotated, as in a fly movement.