When i put DD2 to the boob she will sometimes p...

When i put DD2 to the boob she will sometimes push away from it and scream, or she'll latch on and suck furiously for a few seconds then pull away and scream. While i know this can be common (both my girls have done it) im starting to worry that it will mess up my milk as it started drying up about now with DD1. Not to mention that she is not feeding as much as she was and not having enough wet nappies on some days (not every day) Does anyone have any advice? Appointment booked for friday, earliest we could get in.