I have the same problem with my baby guinea pig bella, the first couple of days there going to be a little shakie because they have to adapt to there new surroundings so it's best not to always be poking your hands in there the first couple of days! Although some guinea pigs are used to humans already so if you get one like that then you lucked out! Basically just let your guinea pig adapt to the new surroundings. But if you can get your hands on them be gentle and don't pick them up so high off the ground. Carefully stroke them on the back to make them feel secure in your arms although some guinea pigs don't like this. GOOD LUCK!