Upvote:2
If you think about it, you actually didn't even see any CBP official when you exited the US by air: the US has no exit controls in this situation (the only officials you saw were TSA, but they only check you don't carry anything dangerous and vaguely that you are the person matching the ID you present — they have nothing to do with immigration, passport control or customs), so nobody could have stamped your passport.
However, the airline will have sent details of your exit to CBP electronically, and they will have updated their records.
You can check the status of your record (called an I-94) on the official I-94 website of CBP.
If your record doesn't show you as having left (which is rare, but can happen), then you do indeed need to clarify this. You should find instructions and contact information for this on the same site.
Note that the situation would have been different if you had left the US by land (to Canada or Mexico, by road or rail or even on foot). In this case, you do indeed need to make sure your exit is recorded when you exit.