What Causes Stretching Feeling In The Abdomen With Bleeding While On Depo Shot?

Hello,I am 18 years old I have been on the depo shot for 7 months (I am not due for my next shot until August) and have never had spotting or a period besides a little spotting right before I am due for my shot. However on Saturday I stretched and had a tearing feeling in my lower stomach then later that day I started bleeding heavily until that night. Then today I also started bleeding heavily. I had had a couple drinks both of the nights before the bleeding. Should I be worried??