Introducing new guinea pigs & suddenly aggressive guinea pigs, help?

[TaylorBreezy]

I recently acquired a new female guinea pig and I need tips on how to introduce her to my current female guinea pig. They will be sharing a cage, if all goes well.

I've also run into a problem with my existing females, who I've had for a while. I adopted the two girls together, but they have suddenly become aggressive towards each other. The two live in a cage, that came with an add on that would allow it to support three guinea pigs. During the day, the two roam freely throughout the entire cage during the day, however I close the gate between them at night. Each pig has their own water bottles, food dishes, hides, and hay-racks. What could be the problem? How can I solve it!
Thanks in advance!
Yes, I've spoken to others and done research to ensure that both of the girls have enough space. They are not sisters, but their previous owners kept them together without problem. I have a vacant cage that I may move one of the sisters two in the aggression keeps up..

 

[Kaede]

One or both of them might just be in heat right now so keep an eye on them, and hope thats all it is. If it is, it will pass. Are you sure they both have enough space? Are they sisters?