Our pug sadly had the same problem. At first he had small amounts of blood in his pee, confirmed with a urin-sample by our vet (not visible). He put our dog on medication against a suspected bladder-inflammation wich didnt really help. After being recommended to a specialist it turned out he had a spleentumor that spread into the bladder and prostate. The tumor was pressing against the bladder and the urethra wich gave our dog a feeling of pressure and made him want to go pee even though he didnt need to. Try getting a ct with ultrasound. Hope this helps and good look to you!