What might be this tiny, tiny invisible bump at the entrance to the vagina?

A Bartholin's Cyst: It is very ikely that you have a barthoin's cyst--it is typically not an infection but can be caused by infection which blocks the gland/duct, thus collection of fluid (swelling) to cause the "bump." this can be size of a pea up size of an egg. Simple incision may do the job, but if infected, antibiotic is needed and cyst may need to be excised. Consult your doc. Good luck.