Do you remember getting injured at all where the lump is? Even if it doesn't seem like a relavent injury. Like you opened a drawer a little too far and it hit you or something of that nature, maybe a pet jumped on you while you were laying down?.... When did you notice the lump relative to the pain starting?
It is not clear what exactly it is, but you need to rule out Bartholin abscess/cyst, which might be infected, consistent with the groin pain which you mentioned.
The Bartholin gland secretes lubrication fluid for the vagina and when the Bartholin ducts' flow gets obstructed, the fluid gets backed up causing this.
I hope this helps.
Best wishes,
Niko