Partial feeding and birth control?

I have been pumping exclusively for my 5-month old for 4 months. I decided to wean him and have slowed down my pumping, but I've been reading into partial or "part-time" feeding, and I'd like to try doing that - where I feed him formula for half his feedings and milk for the rest. My question is - what birth control method should i use? I'm on the mini pill now, but I thought that once you stop feeding you should switch to a regular pill. I used to be on Yasmin, but I'm also concerned about the effects on baby and my milk supply.