What Causes Bleeding Post A Depo Injection?

I just had a 6 month old baby in December 2013. January 2014 I had my first depo injection. I was due for another injection in April 2014, which I didn t go. In the past I have always bled before my next injection && it always indicates that its time for my shot again. I bled for almost 8 days in march 2014. I can t recall bleeding in April but I think I did. May 2014 i bled from like the 2nd to the 10th. I had a small surgery to check for cervical cancer on the 16th and bled for like 8 days, stopped then started back until the end of may. I had unprotected sex June 8th 2014. June 12th I had a brown discharge like jelly/mucus when i wiped &&for about two hours a brown discharge but it wasn t heavy at all. June 14 I had unprotected sex twice. I failed to bled and its only two more days left in the month. I been having pregnancy symptoms for two weeks. I thought my cycle was coming after a trickle from my vagina but it was only clear fluids that happened for two days and multiple times in a day. Once it looked like clear jelly. I took multiple pregnancy test and all was negative. I have been feeling light cramping on and off. Could that brown discharge been my period trying to come on and I tested too early? Or could there be another explanation? I am very fertile. I am 24 with 5 kids but I have been pregnant 7 times and I have used the depo birth control every time.