There are titers done for some of the tick diseases, but they're finding that they aren't very reliable (for humans, for instance, the titers are missing up to 30% of the cases!
) and there appear to be some emergent diseases they can't even test for, yet. So here the vets (and human doctors, for that matter) go on tick exposure and symptoms and may not even look at the blood.
The hints in the CBC can be subtle--low end of normal, or high end, depending on which disease, but still 'normal' and that muddles the picture. Sometimes they want to test blood a couple weeks apart to see what, if anything, has changed, and if indications are clearer the second time, they'll treat then.
There is a snap test that checks for exposure to heartworms, anaplasmosis, Lyme's disease and a canine ehrlichiosis available from Idexx Labs--it's fast and dirty--a drop of blood and results in just a few seconds. It's less expensive than the titers (which require a different sample/test for each organism) and is probably about as reliable. Was it the snap test they used for your girl in mid-May when she was tested for the tick diseases?
At this point it's probably just something to keep in mind if they can't find another cause for the bleeding. If worse comes to worst, perhaps your vet would agree to a short course of doxycycline (which seems to work against most of the tick-borne diseases) to see if there's an improvement. Doxy is cheap but can be hard on the tummy.