What Causes Painful Canker Like Sores On Labia Majora?

I have like two canker like sores on my labia majora (sp?) that are yellowish in color and very painful. Today I noticed they are crusted over. I shaved my vagina a week ago and had sex with my bf that same day and then 2 days later noticed some pain there...I thought maybe it was an ingrown hair or something but after doing research I have freaked my self out.