My Bull mastiff that is about a year and half is throwing up water and she did not eat her food this morning?

She did this a couple of days ago too. She just throws up the water that she just drank. Then a few minutes later she goes back and drinks more water and keeps it down. Then she was not eating a lot later that day. Yesterday, she ate all of her food at both feeding times and was fine. It has been really hot the past few days, and she gets an hour or more walk early every morning (before 8 AM). Except for yesterday, I took her to the park early and just let her run around. Could she be over-heated or worn out from the walk? She is acting like herself other than those couple of times.