Cramping question

Has anyone ever experienced a time when you were standing in one spot and all of a sudden you get a cramping feeling. Then you feel like you are starting your period to find out you didn't. I was standing in the store and all of a sudden a sharp cramping feeling came on in my lower abdomen. Then I felt like my period had just started coming down. When I got home went to the bathroom and what I thought was my period was creamy/watery cm. I think more creamy. Also it felt like my boobs just instantly started hurting. Is this normal? Is this an implantation sign?