Mother in law drama!!!!!?

At my future mother-in-law's house visiting...me and my fiance were on the couch watching a movie and his mom left ..said she was going to the grocery store and he looked at me and said "we're alone, sexy" and we started making out then he picked me up and carried me up the stairs to his old bedroom and layed me on his bed and we were making out and he took off my shirt and I had on a black lacey bra and I guess his mom forgot something, heard me moaning and giggling and walked in and said "what are you doing?, stop!" "omg, please put your clothes on" ...he's 25 and I'm 20 ...we thought she was gone, what should I do or say to her