Unfortunately, it's really important you tell us everything, Mystic Meg doesn't work for us (yet), so you'll have to find this out. When investigating a claim, insurers will always check your medical records, so if you don't tell us about something if could affect your payout.
Here are some suggestions:
Would anyone else be able to tell you what it was (Doctor / nurse / family member)?
Can you remember your symptoms? Perhaps a quick Google of those could help jog your memory?
Drop us a line at [email protected] or start a chat using the icon in the bottom right. This may not be something we can help with, but we'll do our best