It may well be natural, BUT I have to disagree - YOU need to be in control of her herding instincts. It may seem funny her rounding up a guinea pig - although I pity the poor guinea pig - it wouldn't be so 'funny' if she then decided to round up small children or cars or bikes would it? As your collie has a strong herding instinct is there any way you can work with her constructively - sheep trialling for instance?