Consider Bartholin's cyst with superinfection/abscess. See your OB/GYN if symptoms persist.
I had this symptom, and also swollen lymph glands at the tops of my thighs. could not get an insight through obgyn/primary. It was preceded by what I thought was coincidental years of pain, fatigue and nuero symptoms. I had an IUD that seemed to become infected a decade ago which lead to a hysterectomy. The pathogen was painfully slowly spreading as fibromyalgia and the operation didn't come fast enough. I needed antibiotics but it was never a consideration. A decade later I became super lethargic over a few days as I slipped into (pre) sepsis without diagnosis- my heart felt like it was getting minor heart attacks- enough to call an ambulance once a mo... until they finally diagnosed me as septic, but then regretted to inform me that my condition was chronic. I really almost didn't get enough antibiotics to survive. I had to be scrappy. They weren't looking for the right pathogen= standard medicine. Even when I was on watch to drain my heart of infection, no interest in testing beyond the routine tests, which fyi never lead to a Lyme test. I was fortunate to be tipped by someone who had Lyme that it could be Lyme. Treatment was available, privately through an LLMD. No more hard lumps but the treatment starts out rough. Mine were kinda itchy, but mostly just super awful bothersome because gross- wtf is That?? And I had become accustomed to feeling sick. Bartonella cysts can show up in other areas of the skin and internally. Also a stretch mark looking rash is a tell tale sign even if it's cleared since. There are other Lyme types- see an LLMD for appropriate testing. This is not a DIY, money well spent. Good luck!!
I agree with AnnieBrooke. If it were just a bump, okay. You can wait and see but it's getting bigger and bigger by your description. Needs investigation. Bumps under the skin like you mention really are usually cysts. Feels like a pebble under there? You can try a sitz bath to see if it helps. but really, get checked by your doctor.
Can you see a doctor? It might be a cyst and if it has been going on for a couple of months, it might be simpler to have it lanced.