I think that is quiet natural.Most men experience premature ejaculation at least once in their lives when they feel something sexually.Psychological factors commonly contribute to premature ejaculation,it can be caused by temporary depression,stress,financial matters,expectations,sexual depression,afraid of seeing masturbating,lack of confidence etc..
There are some medicines available in the market,if you are concerned too much about this get an appointment from the doctor and ask suggestions.
Or you can try some natural methods such as herbals,holding off by yourself.stop masturbating at least 1 or 2 weeks and see the result.