nipples

Hi I was just wondering if guinea pigs are supposed to have a bald line in the middle of their belly? The picture is of my 2 year old sow. Could it be related to fungal infection? Also her nipples look very flattened, they are sort of stuck to her skin? The picture doesn’t really show it well...

Female guinea pig. Three in June. Only have her and her sister but noticed her nipples look long which I haven't noticed before. Is this normal or should I be concerned?