It's normal for a top on a steel-string guitar to have a slight 'belly' behind the bridge.

Put a straight-edge across the top immediately behind the bridge and you should have a clearance at each end. Add the two clearances together. If they total between zero and 1/2", you're fine. If they total over 1/2", you might have a problem (that's MIGHT), and a good luthier can check it out and tell you if there's anything to worry about.