What should i do if my tip was returned to me by the waitress? Will offend her if i give it back?

On our way to our plantation in Tuguegarao, we stopped over a small restaurant along the high. After eating i left a 50 pesos tip. Upon stepping out the door the waitress called back and gave me back the 50 pesos i left for tip, i was stunned and she smiled at me. i turned my back and sat in the car... I felt guilty i should have left the tip but i was afraid i would offend her...