I would recommend going to see your primary care doctor
or your OB GYN doctor about this issue, especially if things are not clearing up or getting better on their own.
Although what you are describing does not particular sound like a sexually transmitted infection, you are right to consider that and it should definitely be ruled out. The main form of sexually transmitted infection that can cause painful spots inside the vagina is herpes, which typically causes blisters or ulcers. Your doctor should be able to take a quick look to determine this.
If the bump is not actually deep inside the vagina but rather closer to the opening where the fleshy labia are, then another possibility is something called a bartholin cyst. This can happen when one of the lubricating glands near the opening of the vagina becomes obstructed or infected, leading to a quite painful bump that can look a lot like a pimple. Sometimes, these bartholin's cysts may require antibiotics or drainage in order to heal up.
Make an appointment to have this issue checked out as soon as you can.