There are all sorts of possibilities
1. View | Resource Sheet and see what Max Units you have.
Note that the units are what Project will use by default but you can adjust a task's work/duration/units and that can force project to exceed the Max Units which, in turn, will show an over-allocation.
2. View | Resource Usage and then Tools | Options | View-tab and set the date format to something with the time. You may see several short duration tasks that start at (for easy example) 8:00am on the same day. This is particularly true if you have been entering the Start date (not not using predecessor links to let project control the task start).
3. Did you create a recurring task? They (generally) start at 8:00am unless you override it and can easily create a seeming overallocation even if you have followed step 2.
In other words, you created a task for 2 hours and set the start date to Monday. In project, that means Monday at 8:00am. Then you created a recurring task of 1 hour for a meeting every Monday. In project, that means every Monday at 8:00am. The resource has 2 hours of work on the task and 1 hour of work at the meeting. The total is 3 hours but because the two tasks occur at the same time the resource is over-allocated.
I generally never look at over-allocations on anything less than a monthly basis. If the over-allocations are trivial (a day or three over a month) then I wouldn't sweat it. If you have overallocated by 4 days over a month then your estimates are are within a tolerance of 20% -- damned fine estimating, if you ask me.
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