I think transparency is key! If you simply say, “hey, I am human and mess up. I feel like I messed up with your son the other day and disrespected/annoyed him. I don’t think that was right. I got right with him, but you saw it all, and I felt the need to fix things with you, too.” It will take eating some humble pie, but you will earn major points with the MIL! Another possibility is that you thought she became cold, but maybe it was just paranoia on your behalf! Many times I perceive someone’s reaction and assume their motives. And many times I am wrong.