HRM and Nursing - Probably Women Only

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I have a Polar HRM that I love and have been using for a while. I just had a baby 4 months ago and am nursing. I started getting back to the gym about 2 months ago and have been wearing my HRM but I notice that the strap keeps slipping I lose the heart rate periodically. Prior to having the baby, I've never had a single problem with this HRM so I have 2 questions:

Is this happening because I'm nursing and my boobs are pushing it down?

What can I do to fix this or do I just need to wait until I stop nursing?