Medela personalfit?

I can't tell if I need a different size shield for my swing. I'm using the standard size 24mm. I have flat nipples so it's kinda hard to tell if it's too big or too small since my nipples don't stick out. When I pump, my nipple completely fills the tube thing (in width and length) and it's rubbing in the side. This tells me I need a bigger size. But when I turn the pump off I have a small amount of milk drip out of the shield. This says I need a smaller size..
My nipples are a bit sore, I've used it 5 or 6 times in less than 24hours and my nipples have had no stimulation at all for about a week so I expect them to be a bit sore. But I can't tell if I need a different size or not. I've looked at all the diagrams but since I have flat nipples to begin with I'm just like wtf.
Help?

My shields were 24mm at first, and I had the same problem as you, but even though the milk dripped out after pumping, I figured it was normal, got a new size up, and I felt MUCH better.