"Guinea Pigs are very easy to breed. Youngsters are sexually active from as early as 4-6 weeks (females - are called sows) and 9-10 weeks of age (males - are called boars) but mating should be delayed until females are 12 weeks of age. Females come into oestrus (commonly called being "on heat") and are receptive to males every 15-17 days. Oestrus itself only lasts for about 8 hours. "On heat" females are easy to recognise because they arch their backs and hold their rear end in the air. The vagina is usually closed off with a membrane except during oestrus when they initially develop a clear mucous discharge from the vulva. Following mating, a plug of material forms in the vagina and this dries and falls off a few days later."
They are pretty robust - I am sure you will see a small smiley piggy face looking up at you one morning. If it is just one or two, maybe you coudl keep him or her. But get it sexed by a breeder asap - I think incentuous romances are not uncommon [yuk!)