14 year old unspayed jack humping obsession

by Tess' owner
(Kansas)

We have a wonderful sweet loving jack named Tess. She is the best most intelligent girl. She is now 14 never been fixed. She minds us well and is very well taken care of. Good diet and healthy. When it is time she comes into heat she humps all pillows. When we catch her she cowards as if embarrassed and seconds later does it again. Is this a dominance or hormonal thing? We have no plans to get her fixed so what can we do to hinder this behavior. Thank you for any answers in advance.