Is my guinea pig a female?

I adopted a guinea pig named Spike in January and was told it was a male. But I've since become almost positive it's a female. Spike is smaller than both my female piggies and also doesn't have the strong urine odour male pigs usually have (my male Norman is a very stinky boy.) I looked between all my piggies' legs and compared to pictures on the internet. Spike looks very similar to the girls - a straight line with a little dot at the top. When I pulled very gently on either side, a Y shape became visible, just like many websites described would happen with a female. The thing is, Norman looks very similar between the legs as well, except that the little "dot" is more of a pucker, and there is a slight swelling that must be his testes. I know this all points to Spike being a girl, but I just wanted another opinion, because I'd like all the girl pigs to be able to play together without worrying about an accidental pregnancy. Spike was rumbling and trying to mount the other females. Is that just a sign of being in heat or showing dominance? I'd take a picture but I've violated poor Spike enough already with all my examinations.