I’ve had similar experience with both rash & swelling. My swelling was located at the back of my knee & I had a rash all over my body, I was concerned because I new it wasn’t acne which I suffer from too however regarding the rash I’ve been too 3 different doctors and they’ve been telling me different opinions on what this rash is ,I’ve been put on a skin antibiotic for it , it had settled it but it never went away I still have it and been referred to a dermatologist which is gonna take months With the swelling I was told it could be a cyst & was given antibiotics also which helped but I have heard of things like certain types of arthritis can cause swelling there’s also numerous type of lumps it is hard to pinpoint which I’m glad they done tests for you my gp never did i hope you get to the bottom of whatever that is wishing you all the best 💕💕

I have juvenile psoriatic arthritis, my labs look normal, and I get randomly swollen and it goes away. I'd say try to get a rheumatology appointment they have a better idea of what to look for and can provide more helpful ideas of what to do(at least from my experience they do). If you have any questions feel free to dm me!