As Maia said, it's probably most likely lupus is the cause. The marrow test will show if she's producing platlets. I had Platlet trouble a few years before I was diagnosed. Marrow test showed I was making the platlets, so it was assumed my spleen was filtering them out. I was put on 100mg of prednisone, my first experience with pred., a drug I now know VERY well! Pred. did the job and I've never had a return of that particular problem. If the Pred. hadn't of worked the spleen was comming out!
In my case my count was very low, around 1,000. I had a horrible time with brusies. Poked my arm with a screwdriver and my whole arm turned black and blue!