Guinea pigs humping

[Rugrat]

So recently we've noticed one of the guinea pigs humping the other. I know it can be a dominance thing but we're doubting whether they're both females as we were told when we bought them.

I've attached pictures of both of their genitals in the hope someone can shed any light.

 

[Merab]

I can’t tell the sex but I have 2 sows who come into season one after the other and so chaos reigns.
We have wheeking, we have humping and we have rumbling.
Fortunately they generally stick to annoying each other and leave the others alone

 

[Rugrat]

I can’t tell the sex but I have 2 sows who come into season one after the other and so chaos reigns.
We have wheeking, we have humping and we have rumbling.
Fortunately they generally stick to annoying each other and leave the others alone

That sounds like it's a painful time. This is the first time I've seen one on heat so it freaked us our at first, they've been living with us for about 3 months now. Does it happen very often this?

Looking into it now it looks like they are both females. We still have no idea how old they are and thought this might of given us an idea but still hasn't. Both genitals look the same so I assume theyre the same sex.