Depo-provera is notorious for causing irregular bleeding and periods. It takes up to 6 months to get rid of the intermittent bleeding. In some patients this injection also causes cessation of menses. One is protected against pregnancy the day the injection is given. If you do not want to have a child make sure that you get your injection every 12 weeks. When you do decide to get pregnant, it may take from 6 months to 1 year to regulate your menses and get pregnancy.
All of this said, I have seen a rare patient get pregnant while receiving the scheduled depoprovera injections. There is no harm in doing another pregnancy test.