Besides bleeding, what could lower my red blood count?

My blood work in May was all normal - with the exception of a slightly high WBC.

My blood work from last week was all normal - with the exception of a slightly low RBC, hematocrit, and hemoglobin.

The doc said it all looks good and I shouldn't be concerned

I'm not seeing blood.  I know there could be some "mixed in".  What else could cause my RBC, etc to drop?  Also, is a slight drop in RBC enough to make me super tired?

I think you gotta trust your doc on this one and since he says not to be concerned, don't worry. Other causes of low RBC are pretty lousy and you might end up psyching yourself out by researching and speculating too much. I know I end up doing that alot =)
Maybe talk to your doctor again soon and voice your concerns.

Your blood count will vary slightly over time. Your body is constantly making new cells to replace those that are left or worn out. One of the primary responsibilities of your spleen is to filter old, damaged or defective red blood cells out of your system and break them down. If you happen to have blood drawn at just the right point in this cycle, your manufacturing plant may not have caught up with the recycling facility yet. And yes, your monthly period can temporarily lower your count.

Usually you have to be significantly anemic for it to make you feel tired. In that case, your red count would be significantly below the normal range. "Slightly" low or high counts really don't have a lot of clinical significance.

I thought mesalamine as Asacol or Pentasa could cause low iron - I have had that problem myself.