My doctor just ran some lab tests and found a small amount of blood in my urine. What might this mean?

Very non-specific. A small amount of blood in your urine can be associated with a great many things. You should definitely follow up with your doctor on this test result.
Multitude of things. There are many causes of blood in the urine (hematuria), ranging from harmless to serious. If urine obtained during menstruation, there could be contamination from menses. Kidney/ureteral/bladder stones, kidney cysts, kidney/bladder cancer, nephritic syndrome, urinary tract infections, urethral trauma, & various other possibilities exist. It warrants attention and close followup, but not panic.