Breast pain on left side?

For a while now I’ve had this stinging pain in my breast that comes and goes a few times a month and doesn’t stay for long. A year ago I got a breast exam and the doctor didn’t find any lumps so that’s good (I’m 21) I went again to a different doctor and she said as long as it isn’t persistent (which it’s not) don’t worry. My mom told me she gets the same pain a few times a year and HER doctor said it was a nerve. I’m not really worried but I’d still like to know what it could be.... any thoughts?