What Was The Muscle Like Thing Came Out In My Period?

Hello, im Ashleigh and im 13 now. I started my period half way in year 7 and im juat about to start year 9. Just today something had come out of my vagina that looked like a bit of mussele thats about 2 cm long and 1 cm wide. It has really puzzled me and im worried its serious and im to afraid to tell my mum right now incase she says its me over reacting to things. please can you help because its really starting to bother me. Thankyou