Wheever I train my biceps, they hardly get sore. However, sometimes about 3-4 days after my bicep training, I tend to rely on the mirror and perform double bicep poses. Just by doing THAT, my left biceps start to get sore for the next day or 2. The soreness of it doesn't seem like post workout soreness, but rather like I tore something inside my biceps.

Can someone enlighten me here? Why is it my left bicep hardly get sore after I train with weights, but it gets this sharp pain just a day after I flex it hard? Is it something wrong with my bicep tendons?