S.K.
A.,
This is really typical 18 month old behavior. It is the beginning of pushing independence, and it will get worse before it gets better. The "we will get dressed in 5 minutes" works well for children over three, but with an 18 month old, it is just confusing. "Why can I be naked now, but not forever?" With my 18 month old, I get everything out ahead of time and just go as fast as I can to dress her.
Sometimes it helps to have her stand up, hold on to a favorite toy (they learn to switch hands while sweaters are put on pretty quickly) or find some other distraction. Sometimes we sing a song for dressing or diaper changes. After about 50 times, they get that at the end of the song, they are free to go and they learn to hold still and wait for the end of the song.
If you are looking for a reason, children that age are really realizing that they are separate from their mom and want to do things on their own, in their own way. My 18 month old is also more cooperative when she gets some limited power in the situation. She puts dirty clothes in the basket, diapers in the garbage. She can get a clean diaper, wipes and powder, but I wouldn't recommend letting her pick clothes at this point unless it is choosing between two shirts. Holding her up in front of a full closet will likely be overwhelming and trigger a meltdown.
Good luck,
S.