My rbc- 3.92. hemoglobin-11.4, hematocrit-36 while on my period. these tests were normal 6 days prior. can being on period cause a slight decrease?

Borderline anemia: Any biologic test has minor variations when obtained at a different time. Thus it depends on how much the difference is to know if it is important. Since you did not provide the data on the test done 6 days earlier, we have no idea as to whether the decrease is important. If your period has been heavy, you will need to keep a close watch on your iron count.