Everytime you register an opportunity it is connected to a predefined event. It is important to note that the first registration you make at an event is by default connected to the event from your last registration. Remember to check this everytime you register an opportunity.
If you want to change what event your opportunity is connected to simply:
1. Click the opportunity that you want to edit
2. Click the 'Qualification section'
3. Click the 'Events'-field in the top (Could be a name of an event)
4. Change the event to the desired one from the list
5. Your opportunity is now connected to a different event