guinea pigs oestrus
The sow (female guinea pig) has an oestrus cycle of five to six days and is in heat for just a few hours. When mated, the female becomes non-receptive to the male. Oestrus commonly occurs after the sow gives birth of a litter and it is unhealthy if you mate a sow again this time.
"Before breeding a guinea pig always think about how many homeless guinea pigs you can save instead of breeding" Guinea Pigs Club
Caution: Guinea Pigs can sometimes breed at the age, so if you don't have any plan to breed your cavies, it's better if you separate males from females when they don't need their mum anymore so there will be no unplanned litters.