How do I get my milk supply up?

I have a 9 wk old baby and it seems as if my milk supply is decreasing. I am taking a Prgestin Only Pill which usually doesn't cause this and would have weeks ago. I'm also on my period now but I noticed this about a week ago. I pump three times at work during my 8 hour shift. I use to pupm about 6-8 oz a feeding now I'm down to 3-5oz. I am using a hospital greade pump (Limerick PJ's Comfort). Any suggestions to get my supply back up? Help!