Use relatively when you want to make a statement about something compared to something else. After a zero degree day, 30 degrees might feel relatively warm.
This adverb has to do with how things stack up next to each other. Relatively speaking, you could say you're tall if you're around very short people, even if you're just five five. Fat free ice cream is relatively healthy compared to other desserts, but it's still ice cream. If you've been sick for a week and you feel relatively better, you're feeling just a little better. This word only makes sense when you're comparing one thing to another.