I have found that unspayed bitches will fight around the time of their season, My last dog , normally placid became a nightmare when she was in season, and it was pretty scary, I would maybe have a word with a someone with good knowledge of keeping several unspayed bitches but do wonder if spaying one of them will help the situation.

My other thought would be that I wonder if the later dog had gained confidence during the 2 month that she has been with you and was challanging your dogue!!! if this is the case then maybe a behavioirst. You don't need me to tell you but you have to remain in control as it either sense that you are on edge this could well 'upset the apple cart' so to speak, also keep the sticks out the way for the time being!

Hoping and praying that you can sort this problem out
regards
DT