Nervous Mommy

I am 31w+4d and I have been going since I was 27 weeks to get a makena shot every week to help strengthen my cervix because my first baby was born at 35 weeks. Yesterday I went in to get my shot and the nurse had me lay down to get little mans heart rate. First of all let me say both of my kids never had any trouble finding their heart beats until yesterday. The nurse found it but said it was to faint for her, because he was moving to much (& he was defiantly moving alot) and the next nurse that came in found it and jet was slightly stronger but not like normal but she said it was 140 and didn't seem worried about it at all. Everyone is telling me I shouldn't be worried about it but it makes me nervous that it wasnt able to be picked up as strongly as it was all the last times. Am I just paranoid?