I had this - one frank (visible) bleed, ongoing microscopic (invisible) bleeding. I had a cystoscopy, an MRI, the lot: nothing. A year later, I had a UTI then ongoing pain and frequency, but no infection (and again, microscopic blood). Cystoscopy, ultrasound - again, nothing.
I've been diagnosed by my GP since then as having IC (interstitial cystitis) and sure enough, low dose amitryptalline and not eating spicy food or caffeine or fizzy drinks stopped the symptoms within days.
Now, I can eat all of those - but I still take a very low dose of amitryptalline.
One possible cause can be teeny polpys at urethral opening (first time, cystoscopy showed some) very common in menopausal women. Often, no cause found.
I totally get the worrying...but I do think you need to let go of it, unless something changes 