Hmm...the Depo shot (IMHO) is the worst birth control out there. This shot can have symptoms of everything in the book and can last in your system for up to a year. If you are having unprotected sex, then yes you can become pregnant. So my advise if you don't want a baby...use some condoms then. Some women conceive right away after birth control...while others take a few months to a year. So it really depends on how well your body responds to medications.
Hey...as you missed your shot in march, you could be pregnant.
Normally it takes a few months for the depot shot to wear of...but im sure its still very possible if you have unprotected sex. If the home pregnany test was negative, then i would assume you are not pregnant. I suggest taking another test in a few days time, if thats still negative, leave it for another few days. Then re-test. If again still negative, you are not pregnant!
If your really concerned you could visit your doctor who could do a blood test to see if your pregnant. Try not to worry... spk to your doctor.