Help with deadlifts [back issue]

Hi FitnessBlender peeps! Whenever I do deadlifts I ensure that my back is straight with a little bend on the knee -- as what Daniel and Kelly say. However, I still feel a bit sore from my lower back afterwards. I don't even lift that much weight (only 14lbs max per hand). Any tips on how to avoid back pain and make sure that my back is really straight?

Appreciate all of your help; many thanks!