As mentioned, her vulva should be swollen. This is usually hard to miss. The diarrhea might be something else. Or not. Like Debra, I've never heard or experienced this but I have limited experience.
I always feel for them during the first heat. They're still really a puppy, It's all new and confusing, and there are so many hormones to deal with. Sort of like the teenage years on steroids.
You may have this all worked out, but if she's in the house you may want to get a doggie diaper and some wee wee pads. That's worked well for us. Hardly perfect and she'll probably not like it at all, but it's better than stains. As vicyuv mentioned, she will likely urinate more frequently. That lays down the "come hither" scent, so you might want to take her away from the house.