Around a month ago I changed the rear brake pads. For about a week now the inner rear pad has been rattling over bumps, What do I need to do to get it to go away or is it a problem with the new pads?

 

On most cars there's a little spring clip to keep the pads pressed 'forward' (towards the tire rotation direction). But the mazda doesn't have these... I'm suprised there is noise since i've always had to file down the metal pad backplate to fit into the caliper.
You can use silicone on the back of the pad to the caliper piston, which should keep it from moving around... or just wait a bit and rust will build up a bit and keep the pad from rattling