What does it mean when someone says this? "If I start telling you that I love you, tell me to stop, because I won't be telling the truth"

This person has a problem with lying in order to further their own agenda. It was a flippant warning.

Way back in the day girls were warned to stay away from boys who would say those words in order to have their way with them. Sadly, there are people who would use those words in order to score. Then when they got what they wanted they were discarded like trash.