Both Fe and Fi are good at picking up other people's feelings and can be very insightful although not 100% accurate.
Speaking from a dominant Fi perspective I can quite easily pick up people's moods, feelings and motivations. I care about what they are feeling and can imagine myself feeling the same thing. In that way I am affected by their emotions. By easily but consciously putting myself in their place, even if their circumstances or experiences are very different from my own. I am still separate and emotionally autonomous though, I'm not lost in their feelings.
Dominant Fe seems to blur the lines. They aren't imagining themselves in the person's place, they seem to really feel it as their own. I've heard comments from Fe people that they sometimes don't know what they want or feel themselves. They seem to live for others, but in a good way. A genuine way. They aren't self-pitying martyrs, they genuinely care for people, naturally put them first and feel satisfied by it.
An ENFJ or ESFJ could probably explain it better.
And Fe and Fi probably feel different when it's an auxiliary function.