When dealing with govt forms, resist the urge to over-answer, over-share or over-think; just go the most straightforward path.
A patient is someone waiting for or receiving treatment in a hospital - could be ER, could be outpatient, could be overnight. Treatment can be non-invasive, such as being checked and observed for concussion. If medical staff examined you and talked to you, and your parents got a bill, then you definitely were a patient.
On a related note, read all questions carefully, with an eye to advice above. "Have you ever experienced X?" is different than "Have you been diagnosed with X?" The first is exactly as it seems. The second, the key word is "diagnosed" - in the strictest sense, that means a diagnosis by a medical professional can be found in your medical records. That doesn't mean you or a not-a-doc family member decided you "probably have X."
Being honest is always the best policy while answering the question being asked.
What they are trying to get to is have you experienced something that warranted going to a hospital.