One of the key sources of pay-related dissatisfaction is not the amount of pay, but the perception that there is something inequitable about it. Your increase was acceptable until you heard about co-workers who seemingly perform no better than you, but received increases greater than yours.
Although you believe that you perform as well as your co-workers, perhaps your manager does not. It would have been very helpful for your manager to provide you with some feedback as part of this process. In addition, the notion of prohibiting employees from talking about their pay is archaic. In fact, many companies today are practicing "open book management" in which there is more openness regarding company financial matters.
You should talk with your manager about what happened here. One way to do this without looking like you are paying too much attention to others' paychecks is to ask your manager what you need to do to qualify for a better raise in the future. This can open the door to a discussion of your most recent increase, as well as provide you with some steps to reduce the likelihood of having raises that raise your hackles.
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