Piggie behavior toward each other

Question
We got piggies Christmas 2009, both female.  One of the piggies died shortly thereafter and we got another female about a week later.  They've been getting along no problem... until the past few days.  Out of nowhere the younger piggie seems to be doing something to the older where the older is squealing pretty loudly.  It seems to be a noise to tell her to go away and then turns into something that sounds like she got hurt.  We are not noticing any injuries.  I really don't want to cut the cage in half (I don't have room to do anything bigger) and I really don't want to separate them.  Any ideas as to why the younger would suddenly become aggressive?  Or is it the older that's become more annoyed by the younger and the younger is responding?  Help if you can, please!!  I want them to be able to stay together.

Answer
most prob ably it is power play, as the younger has got older she keeps trying to ecome the top dog,it shouldnt really be anything to worry about. but keep an eye when they are feeding to make sure both are allowing each other to eat and drink.

if after a couple of weeks this is still occurng then it may be that they cannot find a balance of power which leaves you with two options, seperate them or get a neutered boar to keep them in check.