Sorry for the long reply in advance! Bolded the most relevant part of my answer.
I do get a lot of swelling. This was actually one of my first symptoms and it started so rapidly (one evening my stomach area just started to quickly swell) that I ran to the emergency because it was so weird. I was sent to the internal medicine ward of the hospital for examination and some blood tests. Everything came back normal and I was informed that I must have some gynaecological problem or must be pregnant. (And when I told them it is 100% sure I am not pregnant, they told me: oh, you just don't know yet. Great start.)
It is most noticeable in my stomach area but it is also everywhere, on my legs, on my face, etc. It is of course most noticeable on my face in the morning, after waking up from a lying position and it gets worse in my legs later in the day due to gravity. It is definitely some kind of fluid.
It also gets significantly and visibly worse in a crash. Visibly worse by looking at my body/face but also visibly worse on the scale, I also always weigh more in a crash.
It is always present on me, not just in a crash, but it has showed a lot of fluctuation during my illness. When I was at my worst, my face and especially my eye area can look visibly abnormal due to the swelling, to the point that I wore sunglasses whenever I went outside to hide it. During my EBV infection, my eye area was also extremely swollen due to edema, to the point that it interfered with my vision for a short time. This is a normal EBV symptom for some people, possibly due to the effed up lymph nodes (although the cause is not quite sure from what I understand). Anyway, I have no idea if my edema is connected to this EBV symptom or not (have my lymph nodes been effed up permanently or is it something else).
When I was 90% recovered once, the extreme water retention was also almost gone. We are talking about several kg of water on a very tiny woman, not 2-3 kg. I need to keep different sizes and cuts of clothes so that I am prepared for every major change.
Another thing is that I have subclinical hypothyroidism due to Hashimoto's thyroiditis (I sometimes qualify for subclinical hypothyroidism - slightly elevated TSH but all the other thyroid values are great - but sometimes I don't and then I'm in the normal range for everything). This was only discovered during the investigations of my post-viral symptoms. For those who are not familiar with this: a lot of people with these values don't even have any symptoms. I'm on medication for it and it does nothing.
The interesting thing is that people with hypothyroidism have a tendency to get water retention and while I was investigated by multiple internal med doctors, no one said it was due to that. Much later after the hospital examination I mentioned above, I was also sent to the internal med doctor of my district for examination and she wrote it on my paper that I don't have any edema (she already knew about Hashimoto's).
Then I was also sent to Semmelweis University, to an endocrinologist/university lecturer there. He gave me the most thorough examination and he also sent me to do all kinds of endocrinological blood tests. Nothing came up, I have great results. I was very interested in the edema/water retention as it is pretty bad in my case. He said it was not water, even though I showed him photos of the fluctuations etc. He said it must be fat or gas. It clearly isn't though!
But I thought it was weird because it would have been so easy to explain the water retention with hypothyroidism and not one doctor did that, they were all quite adamant that it is not water, whatever it is and it is not an issue. This tells me that whatever it is, it is not the typical water retention expected in hypothyrodism and it is not a textbook case of water retention/edema. It must be some idiopathic something. Medicine didn't have an answer for this for me. It surely isn't gas or fat or pregnancy.
