Problem with parents.

I babysit for 2 kids a little boy and a slightly older girl. About a week ago their parents went to a wedding so I was there from the morning and their parents didn’t get back till about 2 so I was spending the night in the guest room. We went to the indoor pool and i swam so afterwards when the kids went down for quite time I took a shower. The 2 kids rooms and the guest room all have one bathroom and i’ve been told i'm welcome to before. I had told them not to come in but i left the door unlocked just in case of some emergency and I thought it was okay. About 2 minutes in they both came into the bathroom and told me they were bored and wanted to eat again. I didn’t know what to do and told them to leave but they wouldn’t and then I yelled at them. They left and didn’t mention it at all but the next day their mom asked me about it and was mad at how i reacted and that I let them see me in the shower. I don’t know what to say back and how to adress it, any advice?