Um, you did say it was on the posterior wall...that eliminates the g-spot. Not to gross you out or anything, but did you need to go number two at the time of your feeling this bump? The vaginal wall is thin and sometimes if you need to poop, you can actually feel the bowel movement through the posterior wall of your vagina. Either way, I would get it checked out, just to be sure. The test your doctor did was for cervical cancer and since your cervix is not located on your vaginal wall, then your doctor would not have noticed it unless you mentioned it.

ok i had a miss type, sorry, the bump is on the anterior wall. And no I didn't feel it then, I felt during my menstral cycle while using a tampon.

Could be the g-spot...but just in case, check with your doctor to make sure all is normal! Good luck.
By the way, the g-spot feels just like your vaginal wall as far as texture goes, but its raised a bit and about the size of a cheerio/fruit loop. If you press it, it makes you feel as if you need to pee. Get checked any way...can never be too safe.