Does pebble sized stools indicated a low fiber diet?

Perhaps: That is possible but if this is persistent, it may indicate other GI problems. See a GI specialist to be sure...

Often: Lots of reasons, including low fiber intake, inadequate fluid, inactivity, meds, colon issues, neurologic issues, other.

Yes, possibly...: Or you don't drink enough water. The antihistamine you are on could contribute also. Constipation is something that can cause many problems down the road--diverticulitis, bowel perforation. Best to always have a goal to never poop rocks or pebbles! i like MiraLAX (polyethylene glycol) because it is safe (draws water into the colon), doesn't taste like anything (mix in any fluid), and doesn't cause rebound constipation.