White stringy substance in Urine

Hey, I’ve had a white stringy substance floating in my urine (noticed it at least 2 times over the last 2-2/12 months- about an inch long or less and sometimes more than one piece) I’ve noticed it when I’ve urinated in a bottle and but it doesn’t seem to show up when going in a normal toilet.

Google says maybe STI’s but have been tested for gonorrhoea and chlamydia recently and both were negative. Could it be a UTI?

Been having on and off balanitis. (Diagnosed)

Urethra has red irritation that won’t heal.

Age 25

Sex male

Height 5’10

Weight 70KG

Race White European

Duration of complaint 2 months

Location urine/genitals

Any existing relevant medical issues balanitis

Current medication none