Extreme aggression from younger of 2 dog!

Hi
New to forums but figured this would be the most affective way to solve this problem.

I have a 1year old female boxer and a 4 year old male miniature dachsund. The dachshund is my girlfriends dog. I have lived with the dachshund for 2 years and received the boxer when she was 4 months. both dogs have always been great with each for the most part otheruntil 2 days ago.Leo(dachshund) can't go anywhere near Myra(boxer) without Myra pinning him to the ground trying to bit his neck. Im extremely scard as Myra is now about 45 lbs and Leo is about.....8lbs. Myra has never before shown aggression towards Leo although Leo has always been pretty to her at times.....bones, jealousy,ect. Is this pack order stuff...and if so why all of a sudden. Note that I am definetly Alpha to both dogs. She also recently got a red irritation on her abdominal,could she just be irritable because of that.(taking her to the vet tomorrow for the rash). Another note neither dog is fixed as I plan to breed Myra. And Leowouldnt be Leo with out his ornaments :/