Help me assess my pain

I'm 24 female, currently facing burning sensation around my tailbone and my left buttock. This is something new and had never faced anything like this before. I have desk job so I sit for long hours but I have been trying to correct it and walk in between and not sitting for long at all. I've been exercising at home too. This pain I'm seeing is from a week or so. And I have not had any injury that I can recall. I'm trying to assess if it's sciatica or SI joint pain or simply some irritation which will go. I'm scared. I've watched a ton of videos including Mc Gill and Charlie Johnson. They've been great but unfortunately the exercises that should be helping to self assess in those videos haven't really helped me. It got me more confused. Because I only had buttock pain and I recently got the tailbone thing too. It worsens when I sit and lie down on my back. Its fine when I walk. I really donno what to do. Any suggestions are welcome.