Two female guinea pigs --- purring at each other?

[Beppie]

Now I am as certain as can be that both our guinea pigs are females.  I just checked again --- and have read all the websites about sexing guinea pigs.

However, their behavior is so strange!  It looks like breeding behavior to me.  They are following each other around, one of them is contsantly purring at the other (as if it was a male), the other one jumps and gets annoyed, etc.

Anyone have 2 females that do this to each other?  Otherwise, my cavys are good friends with each other, until this odd behavior started 2 days ago.

[Breathless Wonder]

This is dominance behavior. 

http://www.cavyspirit.com/sociallife.htm

Safe, non-combative, dominance behavior
Rumblestrutting
Butt sniffing
Butt nudging
Chasing
Butt dragging (they are leaving their scent)
Mounting (any which way: rear mount, head mount, side mount, flying leap mount!)
Nose face-offs (higher in the air wins, one must lower their nose to be subservient to the other)
Teeth chattering: a little (signal of dominance)
Raised hackles (hair on the back of the neck and along the spine)

We had 3 females, and one passed away this winter.  We're now seeing a lot of this between our two remaining girls, as the younger one tries to dominate the other.  We also tried to introduce another cavy, and it was all out war between her, and the younger one of our existing pair.