Blister is hard, red and tender when touched. not open wound. inside vaginal lips. just 1. can it be friction or bc im getting sick? 1 sex partner

Vaginal blister:pain: At age 21, the blister is not likely to be caused by "friction", unless very prolonged sexual activity has depleted natural lubrication. Even with "one sexual partner" , if that partner has had other sexual experiences, it may be a viral ulcer (blister). This may persist for a week or so, and if it does, you should see a physician for tests to be done.