A few tiny light brown "strings" in urine. no pain and my urine was otherwise healthy looking (pale yellow). maybe left-over period blood?

Urinalysis : A urine test would confirm if this is actually blood or just sediments. I hope you are well.

Could be: We can not really answer without seeing what you are talking about. If you just finished your period then left over blood is certainly a reasonable guess. If you have symptoms of a bladder infection like burning and frequency then consider seeing a doctor for a urinalysis