I injured my shoulder while bench pressing? should i just lift fewer pounds?

Change routine: You probably have torn or injured your shoulder rotator cuff, which can be a persistent "plague" on your exercise routine. Do not do the same thing that caused it for a while and in any event exercise through stiffness but not through pain. If this was at a gym, perhaps there is a consultant who can advise you about alternate exercises. You could also see a physiatrist for advice.

No: If the pain persists, there are many things that could have been injured. If the pain resolves, you can slowly return to bench. Even more important is to give it a period of pec and biceps rest, while working on scapular stabilizers. This involves using the opposite muscles of the back, doing reverse butterflies, rows, and scapular retractions. Then slowly resume pec/biceps.

Shoulder injury: Best to relatively rest, and avoid painful activities. Once shoulder feels better, it is probably ok to resume lifting at lower weights, and slowly progress to heavier weights/higher reps gradually. If pain recurs, consult with an orthopedic surgeon.