Depending on the location of the bump, you may be feeling the opening of your Bartholin gland duct. The Bartholin glands, which produce sexual lubricating fluid, are in the lower vaginal wall, one on each side; and their ducts open just outside the entrance to the vagina. Sometimes such gland openings are somewhat enlarged in relation to surrounding tissue. If so, you're just feeling a minor anatomical anomaly that isn't of any concern at all.
That is speculative, of course. But the likelihood of an STD is very low, given the overall circumstances. You might ask your gynecologist is a prominent Bartholin gland duct opening is possible, depending on the location. If in doubt, you may want to get a second opinion. But in the meantime, I wouldn't lose sleep over this.
Good luck-- HHH, MD