You may be having sensitivity to dissolvable stitches. Remember, your lip was operated on, basically, so there will be some pain. If you have discharge and swelling see your doctor.
The burning sensation on the lower lip could be from the numbing medication or from the actual biopsy itself. I would contact your doctor just to make sure.
This is because you have dysesthesia, or an unpleasant or abnormal sense of touch. It is caused by lesions of the nervous system and is totally normal.