Lots of things can cause protein in the urine. Stress, fever, strenuous exercise, these generally don't cause damage to your kidneys, however, there are many diseases that can cause high levels of protein in the urine consistently. If it's a small amount, and just once, I wouldn't worry too much.
Diabetes or hypertension (high blood pressure)
Immune system disorders
Before I became a diabetic I had all the low sides of diabetes such as low blood sugar, so that could be what's going on with you. When they found Protein in my urine I was teenager and look my age now. I am way older.