Can you write me a gentle apology to my boss about how I can't watch her cat next weekend as I am highly allergic? Write it so that it can be sent as a text message to her.  Let her know that I would if I could!
Hi [Boss' Name], I'm sorry, but I won't be able to watch your cat next weekend as I am highly allergic to cats. If I wasn't constantly sneezing, I would have loved to housesit for you!