sounds like pseudo-hallucinations rather than delusions.
(you have insight fairly quickly into the fact that they aren't real)
do they tend to happen when you are feeling very anxious / wired??? that is relatively common... i would say that emotional stress induction would be a more likely explanation than head injury.
do you have olfactory hallucinations (funny smells)?
how about seizures?
(thinking a little about TLE - temporal lobe epilepsy)