C.O.
A. - this happens...it's a manipulation technique that babies use....they have learned that through crying or screaming - they can get their way....
When changing her? Just tell her "the faster we get this done- the faster we can get back to play" or you can give her something - even ask her to help...at 18 months - many kids, not all, like to help....they are walking and independent (or at least that's what they'd like to think!!) so instead of fighting over it....you have options...
1. you can make it fun - give her a toy or something to play with.
2. you can ask her to help...get the diaper, wipes, etc.
3. you can be firm and stay in control of the situation and NOT let the control be handed over to her...you can be nice and still be firm.