Question

Ok question..

So I've been pumping and freezing my milk since I've had my baby and just yesterday I notice that my milk tastes sour after I thaw it and getting ready to feed him.. I've tried it myself and it make me vomit so I threw it out so now when I pump I taste my milk before I put in the freezer and it tastes fine and he latches on fine. So I'm really confuse I thought I was storing my milk correctly am I doing something wrong?