What might cause a puffy/jelly-like sac (notable fluid buildup) along the occiput?

[BMW21_]

Medical Case Question..

I want to start off by saying I've seen my GP who knows me well and am scheduled for an ultrasound, however, he's as stumped as my physiotherapist & I are so am turning to this wonderful community in the off chance anyone has experienced something similar, knows someone who has or is in the medical community & has any thoughts or next step recommendations.

I'm in my 30s and am relatively healthy but do have some spinal degeneration that is mostly under control, sharing in case there's any relevance. The last 3 weeks I've had very strange things occur, now because of previous (rather significant) gaslighting by numerous medical professionals because I'm "too young to be in that much pain" or "to have spinal degeneration" etc despite imaging finally done provibg them wrong it's often a little nerve-wracking when something new pops up.

New circumstances that have me confused & am not sure where to turn next:
- Physiotherapist who has treated me on & off for 4 years advised me during a session 3 weeks ago (went to see him for what my spinal specialist suspects is occipital based pain/headaches) "Notable fluid buildup along the l-occiput" (base of my skull) and that it feels like a "Jelly/puffy-like sac" that's not hard but is a formed lump-like texture stationary/barely moves a bit and doesn't at all feel like normal tissue but also can't properly feel the C1-C2 joint because of it.
- It doesn't hurt to touch when my physio is assessing it. The fluid buildup/puffy/jelly-like sac hasn't changed in approx 3 weeks
- In the area immediately around this sac it feels almost like there's small voids that shouldn't be there.
- I do not have any sign of infection, psoriasis etc
- There is no visible deformity
- Wthout trying, I've lost 11.6 lbs between March 14-Aprill 3, 5.5 of those lost lbs were between March 23-April 3. This is quite abnormal for me especially since I'm not actively trying to lose weight. I'm not obese but probably could lose another 15-20 pounds but am happy with my weight & body currently.
- Sporadic low-grade fever approx 5 times & typically only in the evening the past 3ish weeks. When it occurs it's usually around 99.2-99.5
- My sensitivity to both light & sound has increased slightly over the past couple of months.

I don't know if the latter 3 points have any relation but included them as they occur within a similar timeframe that this strange fluid-lol Ike sac appeared

I believe the purpose of my physician sending me for an ultrasound of the area is to see if it's cerebral spinal fluid but I'm not convinced (I know my body well). Providing those results come back normal, would there be anything to watch for? Would an MRI and/or X-Ray be benefit?

TIA for any thoughts you may have! I'm doing all the things I need to be, including taking everything I read online with a grain of salt lol

[GuitarRox]

I sure hope you've gotten some help from your doctor on this.  There is a type of cyst that came to mind but it is usually in babies. So, that is kind of hard to imagine. They are caused by blockages in the lymphatic system which seems like it 'could' theoretically happen to anyone. What did the results say? The other thing I wonder about is chiari malformation (there is a great forum for this here on the site under communities). https://www.mayoclinic.org/diseases-conditions/chiari-malformation/symptoms-causes/syc-20354010