Answer: The vagina does not have a smooth surface. It is not like the surface of your tongue. It may be like the roof of your mouth, only soft and spongy. It is actually much like a series of folds or bumps of skin. (See the two photographs shown below.) I would guess you are noticing one of those folds or bumps. They come in different sizes and shapes. I would not worry about it unless it become sore or changes in size or shape.
Depending on how far inside you it is, you may be able to see it with a mirror. In the future you may be able to buy or get a speculum and have a close look. A speculum is a device a doctor uses to open the vagina to do a visual examination and to see the cervix. They sell plastic disposable ones for less than a couple dollars, if they do not just give them away. You can ask your doctor, gynecologist, or nurse for one.
Depending on how far inside you are inserting your fingers, you could be touching your cervix.
Please continue to explore your body and ask questions.
From the book A New View of a Woman's Body, Copyright 1981, The Federation of Feminist Women's Health Centers, Illustrated By: Suzann Gage, L Ac, RNC, NP, ISBN 0-9629945-0-2 Pbk.