Hello and thanks for your post,

I can understand that finding a lump however small can cause a lot of worry. We can't diagnose you but it may well be something called a Bartholin cyst which are fairly common  small fluid filled cysts that sit just inside the vagina.

I would make an appointment to see a GP and they will hopefully  help  put your mind at rest. GP's  are used to being asked about all sorts of lumps found in intimate places and I am sure will put you at ease. You could always ask to see a female doctor if one was available. 

The NHS has some information about these particular cysts which you can see here

I hope this helps.

all the Best,

Catherine