I would get it checked out by the doctor....but it could just be the watermelon. I remember my little sister ate a lot of watermelon when she was 3 or 4 and went to the bathroom to have a BM and it look like blood had come out. When my mom examined it further, she realized it was just watermelon. It could be that and some maybe got on her underwear.
My girls eat TONS of water melon - it def could be that, esp. if there are no other symptoms.
it is better to check with the doctor, but it could be watermelon, I would smell it, sorry it sounds awful but blood smells diffrent than watermelon.