“Seeing something that was not there?”

Dear Madam, 

You said it happened only once. So, how come you are still dwelling on it? What was the bad environment? Did you go through a surgery? Who had access to you to give you a drug? Are you on any medications? In case, you are who prescribed them? Have you consulted with them regarding that one time experience and side effects of any medication you are on? I would suggest consult with your PCP and in case they advise to see a licensed psychologist, then follow through and resolve the issue so you will have peace of mind and can then move forward and enjoy your life. 
Take care and remain safe. 

Dr. Lata Sonpal

Yes and no. Hallucinations (seeing/hearing things that are not there) can be caused by many different things: One - could be drugs reacting in your brain, another could be depression, or could be some types of mental disorders (usually comes up between ages 20 and 40). If it only happened once, and you were not depressed then, just make a mental note of what type of medication you were on, and observe if it happens again in the future under the same medication.

View this link for some insight as to helping you understand your question:
https://www.webmd.com/brain/why-am-i-seeing-things#1

Your are very likely right. Sporadic visual hallucinations (in contrast to frequent auditory hallucinations) are usually associated to drugs specially anticholinergic ones; even OTC ones like Benadryl, Delsym, other cough and cold remedies.