Father in law accused me of being sexual in front of my kids… how to handle this?

My 4 year old has his own iPad for his sensory processing issues (autism). For safety, he also knows his emergency contact numbers, which are my parents and my in laws. He likes to FaceTime them randomly and talk to them. The other day he FaceTimed my father in law and was talking to him about Elmo. He said “let me go find my Elmo to show you Papa!” and he left the iPad propped up against the wall. In the background, I was exercising, which I do mild aerobics, Pilates and strength building. I was in the middle of a glute bridge set which looks like this:

I heard the FaceTime call end before my son returned with his Elmo. I didn’t think anything of it and just thought he got busy.

Fast forward to that evening. He called my husband and told him he saw me do “something disturbing” on a video call with Aden (our son). My husband was like “um, what?” and my father in law said I was humping the air and masturbating. 🫣

My husband asked me about it and I was like “um what?! I was fucking exercising!” My husband laughed and then explained that to his father in law.

HOWEVER, my father in law did not believe him and said “I saw what I saw”. And now he will not return my husband’s calls or my son’s calls for that matter. I’m really dumbfounded! I’m afraid he is going to tell someone I’m at home masturbating in front of my kids or something! I’m also afraid he’s going to tell the entire family and they’re basically going to gossip about me being some kind of pervert. I’m a sexual abuse survivor myself, having been molested as a child, so I’m really triggered by all this and don’t know how to handle it emotionally. Wwyd?! 😭