Red hot knees?!

When I was 15 I went white water rafting in Malaysia and burnt my knees pretty bad. There's a long story (including running for a train and being more worried about getting a tampon in on time) as to why I couldn't put sunscreen on but it was basically a series of bad timings that meant they got burnt.

At the start i thought it was funny I had only burnt my knees and made it into a joke because I assumed it would just heal and no longer be a problem. Four years later it's not funny.

I'm now 19 and kinda worried it might be linked to the sun burn or just that I noticed it because of the sun burn and it was a pre existing thing but my knees get red hot sometimes and I don't know why. They're not painful at all but they're super hot (I can feel it through jeans) and I'm really pale so super self conscious about it. I've not showed my knees in years because of it :/ the photo is after being stood up in the kitchen for half an hour and standing is the only thing I've really noticed that directly causes it. I sometimes get it for no apparent reason though.

I can only laugh it off to an extent because it's killing my confidence, hiding my knees is impractical in hot weather and I'm fed up of maxi skirts 😂

I initially thought arthritis but I thought that included pain and this is completely painless?

Had anyone ever seen this before?