What is causing my right leg and my lower and mid back to have constant soft tissue swelling for no reason. I've been having this problem for 8 months?

Odd. Chronic swelling lower right back/leg of twenties female most odd. I assume "swelling" means discomfort. A loose and subluxing sacroiliac joint can often cause pelvic obliquity in lean females and also be related to unilateral sciatica and low back discomfort. Joint hypermobility syndrome (see brighton criteria), benign form mostly in females, can potentiate this type musculoskeletal disorder.