Welcome to Health Care Magic.
Firstly using Zyrtec
has got nothing to do with blood in stools.
Presence of bright red colour blood in stools is suggestive of fresh bleeding mostly from anus or from the perianal area.
The most likely possibility is from perianal region in the form of fissure or a small area of skin rupture. This is common among children due to hard stools ( remember the child need not have constipation
even hard stools can lead to fissure.). In that case giving her high fiber diet or a stool softener
( only if the age of your child is above 2 years)
You have mentioned hat she passed hard and soft stools.
While passing hard stools the anal skin and sphincter stretches and skin might have ruptured leading to the blood staining in the subsequent soft stools.
Just examine the anal region, if there is a cut then the cause of bleeding is likely to be due to fissure .
If there is no visible cut, then the bleeding could be from the anal mucosa. Just observe for 2-3 days, if your kid still has blood in stools better see a paediatrician and take his advice.
Hope your child is healthy.