J.B.
Hi K.:
I was a die hard breastfeeder for the first 8 months and, just like you, my supply dropped dramatically starting around 7.5 months. I had read that this happens as your baby grows older, althouth I thought it would work the opposite way -- the bigger she got the more supply I would have. We started supplementing with formula around 8 months and by 10 months she was not interested in nursing at all, so we stopped. I didn't want to give her formula at all, but it turned out to be fine (if not a little expensive). If you're totally opposed to supplementing with formula, I would turn to your local La Leche Legue, in addition to any advice you get here. The hot days may be dehydrating your body causing your milk supply to drop. Drink more water! There are also supplements you can take to increase your supply.
Also, that diaper rash could be heat rash -- our baby, who has never had diaper rash, just got a bad case of heat rash and the doc said to let her walk around with no diaper to help it air out throughout the day. Just a thought!