Ok these are some things you need to check before assuming super-natural reasons for the maggot attack.
Are you on medication, prescribed and un-prescribed? If so, were you on any medication that day? What did you eat and drink on the day of the maggot attack?
Have you ever done chemical drugs for recreation? Even once?
At what time of the day did this happen? What were you doing when this happened?
I don't know what spell you cast and why you feel it backfired. But do revisit your thought process while performing the spell. Did your mind lose focus and wander off for a bit? Did you feel fear or a sense of negativity?
If you've cast a spell that you know you shouldn't have, then it could be sub conscious guilt that's making you see things.