Nitrofurantoin - Would it be risky to have sex a day after taking this antibiotic?

Antibiotics do not alter the protection you get from your Depo Provera. As long as you've taken your Depo shots on time, you will be covered.

If this was your first Depo Shot:

If you had your first shot during the first 5 days of your period, then you did not need to wait before you were covered against pregnancy.
If you were outside of the first 5 days of your period when you were given your first Depo shot, then you needed to wait 7 days before you were covered.

Once you follow those rules, you are covered by Depo for 12 weeks, regardless of any bleeding you experience in between shots.