strange large lump, very worried

Hi,
I'm 18 and I'm nearing the end of my period, everything seemed fine, I haven't felt any different at all but as I was inserting a tampon I noticed a large lump on the inside of my vagina near the top though I can feel that you can get past it. The lump is quite large about the size of half a golf ball and rather distressing for me as I've never noticed any such thing before, I didn't notice it during my last period (I use tampons). I'm just worried whether this is cancer or something. I would describe it as a kind of nub, its not painful to touch. Though I've been trying to inspect it a little more it actually has caused mild cramp feelings. I've never inspected myself before and I'm worried I might have missed it growing.
Any advice would be greatly appreciated.
Thankyou
p.s. I do sound rather anxious in this question, so sorry I've just never experienced this before xx