There is no classical symptom as foamy urine in urology
practice, but it is more often than not related to kidney diseases. One of the causes of foamy urine has been stated as kidney disease leading to excess proteins in the urine. It is not always true and is correct in only 20-30 % of cases.
So rest 70 % of people will have no kidney disease or no reason for this.
Since your ultrasound and urine tests have come clear, we can assume you come in remaining 70 % .Never the less it would be better to have regular tests every 6 months just to be sure.As long as the tests are clear there is nothing to worry. This is maximum you can do from your side to stay clear of problems. Worrying would not help a lot.
Basically various factors decide foaming like Ph of urine, concentration etc. Holding urine for long time can be a cause sometimes as longer it is stored in the bladder
, more bacteria can proliferate. Drink plenty of water. Try Vitamin C supplements which can alter the Ph of the urine and may resolve the problem.
There is no prescribed treatment as this condition is not a disease itself.
If you have any doubts, please let me know.