Thanks for posting on HCM
One of the side effects of depo shots is abnormal menstrual cycle or spotting and can cause other side effects. Depo shot protects against pregnancy
in more than 99% (chances of failure are significantly less than 1%) but does not protect against STDs so you are probably having an acute pelvic inflammatory diseases or having another pelvic diseases which would need to checked out by your gynobs. Rarely there is a possibility of extrauterine pregnancy which if ruptured might present as you described.
My suggestion is you should seek medical attention urgently. One can never know exactly what can be going inside. Emergency pelvic ultrasound
can reveal a lot of information regarding the problem. If no major issue, your doctor will reassure you and you can go back home with a peace of mind.
Hope this helps