Hello and welcome to HCM,
Thank you for your query.
I doubt alka seltzer will cause the bleeding as it is usually not related.
There is a chance that the cause of bleed could be different. In normal, the most common cause is some obstruction or abnormality of the renal system.
Eg. Stones - once these stones grow in size, they can get lodged at a place and cause mild bleeding
Tumor- due to growing size they can put pressure on the renal system and cause the bleeding
The best thing now would be to consume as much water as possible, even if it means passing urine frequently.
The next thing you need to do is consult a urologist
and get an ultrasound
done, to confirm the diagnosis.
I hope I have succeeded in answering your questions, you could write back to us anytime for further clarifications.