can you tell me why...

Someone tell me why...
I spotted a week ago but my breast are sore and I feel bloated all the time. I am on the depo shot 3rd round. 
I am sexually active but we had unprotected sex like a month ago but I don't believe I am pregnant because I am on the depo. 
I just don't understand why I am having symptoms like sharp pains in my breast or cramps and I have been having a weird discharge that is like light yellow/white 
Any thoughts 

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