My toddler vomitted for several hours yesterday without fever nor diahra. Today she is back to normal. I wonder what illness could she have?

May not have been il. A kid can certainly have a brief vomiting episode from an infectious trigger & viruses are most common. However, given toddlers interest in exploring the world & using their taste buds to test their curiosity; this could have been related to ingesting a foreign substance. Anything at babies level;potted plants with bugs or moss, etc. I've seen amazing things end up in their poo over 3 decades.
Vomiting. Many possibilities. One caution---check the house and be sure the toddler is not exposed to anything that could be toxic. Be sure the cabinets are locked etc.