There is a large bump on the lower part of the vagina close to the vagina hole......It seems to be something under the skin. Could it be serious?

See doctor. This is likely benign, either a sebaceous cyst or a bartolin's gland cyst. In either case seeing your doctor or gyn can quickly answer the question and get it better.
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.