Baby didn't wanna walk

Hi. My baby is 15month and have been walking since she was 10 month. One day she suddenly didn't wanna walk when we put her down on the floor. I tried to get her to walk and come to me with her favorite things, to check so this wasn't just somthing that she had set her mind to, and she really wanted to come but as soon as she put her feet on the floor, her face was pain and she sit down. We went to the doctor and made x-ray and all sorts of tests, but they couldn't find anything. But they gave her antibiotic and after taking it she got better and started to walk again, but we could never get any answer for why this could have happen. Is there anyone else who have experienced this? Thanks.