It's probably NOT your body trying to wean. It doesn't really work that way.
Do you think your body is about to start having menstrual cycles again? If this is the case it can definitely hit your supply hard. I dealt with this since my daughter was 4 months! To combat this: When you ovulate, start taking a calcium/magnesium supplument (2000mg calcium/1000 mg magnesium). This will help maintain your supply. Here's more info: http://www.kellymom.com/bf/supply/low-supply.html#causes
Also, if you're pumping 5-6 oz, you're probably pumping enough. How long are you away from your baby? Here's info on appropriate ammounts to send with your breastfed baby. Remember, breastmilk and formula are very, very different. You do not calculate breastmilk needs using weight and age since breastmilk grows and changes with your baby. Here's more info on that: http://www.kellymom.com/bf/pumping/milkcalc.html
Hope this helps! She probably doesn't want to drink the formula because it tastes bad! :)