What Causes A Lump Inside The Labia Minora?

I am a 25 year old female who is sexually active with my partner. We do not use protection as we are monogamous but have both been tested and haven t had any cause for alarm. I have been fighting the flu/sinus infection and been on two courses of antibiotics. I woke up the other day with slight discomfort in my pelvic region. I noticed a small, dark reddish/purple bump on my inner labia. It doesn t hurt but does provide some discomfort when sitting/walking in tight clothing. It seemed to show up over night! I have been keeping the area clean but am worried as to what it might be.