Assuming you did not actually see the blood exit your body, blood found in the toilet bowl may be disregarded and does not render you niddah as it is a surface that is not susceptible to tum’ah.
The fact that the technician said you are spotting does not indicate you are niddah since it’s possible the spotting she saw was caused by vaginal/cervical trauma from the ultrasound wand.
You should continue taking precautions against becoming niddah from any further staining by wearing colored underwear and waiting a few seconds after urinating before wiping.
Please see our articles on stains and toilet paper for more details, and feel free to get back to us with any further questions.