where's my core?

I've been going for Bikram Yoga for the past week. I must have attended about four 90 minute sessions so far. It's been great.
The thing that's been niggling at me since I started going is how not-easy it is anymore to just focus in the moment. I used to think shavasana meant I could daydream and just let my mind wander. This yoga teacher says you need to focus on your intent, realign those and just concentrate on why and wherefores of the reason behind you doing yoga today and what it's doing to your body, what you need from it to do to your body etc etc.
I also was amazed by how restless I was. My body was tired and welcoming the few seconds of breaks between the asanas but I just could not chill for want of a better word. Lol bcos Bikram yoga is done in a heated room to make you sweat. So you get what I mean. Any second that I got my mind was ticking off things to do, things I did, even something as silly as will the kids enjoy the lunch I packed today for them. The thought comes in unbidden and some other part of my brain is telling me incredulously- 'are you serious'? You're actually concerned about things you know for sure you needn't be?' It was like becoming a mother or just being at this juncture in my life didnt affford me the luxury of aimless floating. I wondered - why? and is that true for all moms?
I mean I know we worry but when you know you're not needed, the kids are in school, the husband's at work, the house is locked, yes you checked and double checked the burners were off and the iron box was off and all the windows were shut then what was that niggling sense of something must have bypassed my notice and am I gonna find the the police, ambulance, firefighters all surrounding my house? Maybe someone from the school called and I cant use my phone while class is in session and its an emergency? What just happened here?
I don't like it. Does it come with being a mother package and does that mean I'm never going to be able to be carefree again? eeks! no no no!
The funny thing is - when I'm at the gym, it doesnt bother me. It's only those quiet moments when nothing and no one else needs my attention. It really surprises me. I might have been like this for a while but being forced to be quiet and sit still at someone else's prompting caused me to notice it.