Like Lizzie said, the heat is definitely a trigger for MG. Try not to get over heated, or do too much when it is really hot out. I just generally cut back on everything I can during the summer months. This is just one of the things you will now have to watch out for. Check out the many good sites posted in the sticky at the top of the MG page. One in particular which I found helpful is http://www.myasthenia.org.au/html/lifestyle.htm which just happens to be from down under!
Hugs,