Well...the two are NOT remotely the same at all! Diarrhea is VERY, VERY watery stool that the dog can not usually control when it comes shooting out of their rear. When in heat, the vulva is usually rather large and swollen looking and the bitch may lick it with a fair amount of regularity. The heat cycle runs like this: The cycle is the same for all breeds. 1. Pro-estrus (swelling of the vulva, clear discharge that turns bloody) 8 to 13 days. 2. Estrus (normally bleeding has stopped. Vulva less turgid, but still swollen. NOW is when the bitch is receptive to breeding most likely and to conception) 4 to 7 days. 3.Metestrus (when puppies are growing, if successfully bred. Also time with false pregnancy can happen) 6 to 10 weeks. 4. Anestrus (quiet period of variable duration when there is no activity in the bitch's reproductive tract) 15 weeks. If your bitch is having highly watery stools, for more than a day, you need to start worrying about her becoming dehydrated...and that usually means GET HER TO A VET! The Vet will put her on an IV of liquids to prevent her from dehydrating...which if not done, could lead to other medical health complications that could threaten her life, and cost you a lot to try and revive her...so don't piddle around with it, if she is having extreme diarrhea! Likewise, if her cycle seems off somehow to you, have her checked out. It is possible that she has managed to get an infection, which can also lead to dehydration, and other issues internally. Better Safe, than Sorry!

If she was having her cycle, you would see blood, in small amounts, but you would notice. Diarrhea, is just watery poop.