I've not ever had a dog with morning sickness... I would tend to think that something else may be going on. As she's only just done her heat, her body would really not be going through much change should she be pregnant.
Just out of curiosity.. if you did not wish her to be bred.. have you not considered neutering the male or are you planning to breed down the road? I just find it so much easier to not have to monitor activities amongst my dogs with regards to pregnancy. And of course, non of mine are champion lines that would warrant breeding... just PB rescues with no lineage to speak of.