What Causes Swelling In The Legs And Urinary Infection?

I'm 28...my leg is swelling since last year...as per Dr. advice i did some test where urine infection was identified. now last 3/4 months this problem is arising time to time...specially at night....this year Dr. gave some test and again urine infection has been identified but other tests were ok....but still now i dont know what's the actual reason behind this swelling in leg...feeling very irritating as i cant move flexibly when this problem arise.