My parakeets head movements?

i just gave my parakeet some new food to try. i got them to eat lettuce, strawberry, and watermelon. so today i tried cantaloupe and wheat bread and it really seemed to like it. then it stopped eating for a little bit and kept bobbing it's head up and down when i was watching it and it wouldn't stop like it was having a bad reaction to it. i read though they could do that when they are happy too. it stopped now but what could that head bobbing mean?