Yes it is so normal it has its own name. It is called a queef and has a variety of other names to boot. It happens when air gets pushed into the vagina and it has to come out eventually. The most common reason for it happens is the guy pulling out too far during intercourse and then air gets pushed in. So you can tell him not to pull out all the way during intercourse and that should help.
But honestly it happens to us all at some time or another and nothing to worry over.
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Married 3/25/95
DS 13 yr
DS 8 yr (3+ yrs ttc)
TTC #3 since 01/02
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VERY EMBARASSING 
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