Sounds like something that happens to me once in a while. If I dress too warm on my legs and then ride anyway, I get what I think is a heat rash. It's red blotches, that look a little like a bruise. If I keep on riding it takes a long time to go away. I have arthritis in my knees and hips, and ankles etc. My joints feel good if I am dressed to keep my legs very warm, too warm for most people. I also wear long underwear under my jeans, this does not help either.
If I stay off the bike for two days and don't get too hot and sweaty, it goes away. It looks really terrible sometimes.