help please ???

so today this came out of me and i have never had anything like this happen while on my period. It was a little sack / baggy looking thing like you were able to fill water up to the top of it and it will hold it. First picture is how it came out at first so I decided to stretch it to see better but also I’ve been getting really bad back pain. Does anyone know whats going on? I was taking my birth control pills but i missed two days on purpose because i didn’t want to take them anymore and kind of dont plan to