First of all, you need to check your pregnancy
status with home pregnancy test or blood test
to make sure.
Taking depo shot also cause the same thing as side effect of itself. You need to consult with gynecologist to get examined once to proceed other investigation if required. Take suggestive measures accordingly.
Take healthy diet with vitamin & iron supplements
. Practice protected sex for 2 weeks after taking depo shot. Maintain genital hygiene properly. Good luck.