My urine is foamy some times. I was affected by nephritis 7 years before

[aruns327]

I was affected by nephritis 7 years before (Due to some infection,through wound), Treatment was perfect and at perfect time. After that  my urine is foamy some times (may be evening,morning    like that).If i drink lot of water, then foam is not there.
I have done lot of tests including blood urea, creatinin,urine albumin......a lot of tests at least twice in year.
The results of all tests are normal till now.

I don't  have any  other symptoms of kidney related problems.

What may be the possible cause of foamy urine.