Behavior I think my piggy might be pregnant?

About a month ago I discovered that one of my guinea pigs (that I thought was a girl) was a boy. Up until then I had been keeping he and my girl in the same cage, so there was plenty of time for contact. I separated them as soon as I could, and hoped for the best. My little girl is just over 3 months now so she's still growing and maybe that's all but recently I've noticed she's getting a little more irritable, and she really doesn't like being picked up anymore. She also seems to be gaining weight, but when I pick her up it's her tummy that feels like it's getting bigger. I know that guinea pig pregnancies take around 60-70 days, but she has been separated from the boy for about a month and I'm just starting to see changes now.

So, I'm wondering if it sounds like she might be pregnant? Or what are some usual signs of pregnancy in guinea pigs?
And, if she turns out to be having babies, how do I take care of them when they arrive?