So embarrassed 😳😔

My fiancé and I were visiting his parents, as we usually do a couple times a year (we live in a different state). Now his parents are a bit conservative with regard to their opinion of how non-married couples should behave/live. So of course they had prepared two separate bedrooms for us before we arrived. Now for some context, my fiancé and I have lived together since last May. But because it's their house we respected their wishes and didn't protest the whole separate bedroom thing. The whole visit went great and this morning we were getting ready to leave. The shower in the bathroom we had been using has a pretty clogged drain, so neither of us really wanted to use it and then my fiancé said let's just use their shower. Now keep in mind we know both of his parents work schedules, so we thought we would be perfectly safe because his mom doesn't come home for lunch until noon. However, she sent him a text saying she was coming home because she didn't feel well, but we were already in the shower. As we're getting dressed he sees the text and at that point it has been like 20 minutes since he got it. His mom's work is only ten minutes away, so we were rushing to get dressed before she got home. But then we noticed that her lunch cooler was already on the counter, so we knew we were too late. We don't know exactly what happened, like did she just hear the water running above while in the kitchen (knowing it was coming from their bathroom) or did she actually come into the bedroom and see us in the bathroom? Either way she was gone and did not come back. I feel really bad because we were in her space, but we weren't expecting her to come home early! We did our very best to respect their wishes during the whole trip (like we always do), and we had meticulously squeegeed the glass portions of the shower so as to not leave a trace of our presence. I guess we had the mindset of "what they don't know won't hurt them". But it didn't matter because our careful plan fell apart. After we realized that we got caught, my fiancée tried calling his mom but she ignored him. Then later she sent two texts, one telling us to drive safely and the other said that she couldn't find any words (in response to walking home to hear/see/notice us) and that when she did they would talk. I understand that it would make her a bit uncomfortable, but I'm a little confused by the severity of her reaction because she has called me his wife several times before. It's like her way of coming to terms with us living together is to just consider us already married. Which is fine with me because obviously I am committed to this man for life and love him dearly. So if in her eyes we're basically married already I don't feel like it should be THAT big of a deal. What do you think? Am I just a bitch for taking a calculated risk and agreeing to shower with my fiancée in his parents shower? Did we get what we deserved? Or is she over-reacting a little? Has anything like this happened to you before? Please tell me what you think, because right now I just feel like a piece of shit. 😔