This is the 44th most frequent Bulgarian word.
когато
"Когато" means "when" in English.
In this sentence, 'Когато' introduces a time-related clause describing the condition or context under which the action occurred: specifically, the time frame of being little.
Когато бях малък, играех в парка.
When I was little, I used to play in the park.
Here, 'когато' introduces a subordinate clause describing the future time at which the action of calling should occur.
Звънни ми, когато пристигнеш.
Call me when you arrive.
In this case, 'когато' is used to indicate the recurring condition under which the main action takes place: specifically, when she feels happiness.