I am in your same boat as far as the bad and the good days... I am 33 mother of a 3 year old and a 17 month old. It is crazy times...
I have played competative sports my entire life and due to injuries my body just can't do what it used to. It is hard to reign it in. I am learning as I get older that things are changing and I just have to adjust to them and find new ways to feel the edge. (legal ways)
Due to ankle injuries I have now taken up road cycling and have fallen in love.
Just remember your body held twins for 9-months. It takes a while for that to come back. Most importantly is to listen to your body.
I am glad you are going to get the heartrate checked out. Unlike men, women actually go to the doctor when their mind tells them to! Please let us know what they say and give yourself a day off.