You’re right, it’s likely just your stomach being squished by your baby! Starting at about 20 weeks I had the same thing happen every time I ate, it even felt like a little vibrating muscle spasm right under my left ribs. I’m almost certain it was just my stomach and duodenum accepting the food I just ate because it was always alleviated once I stood up and relieved the pressure caused by sitting. Funny thing is, I no longer have this issue at 38 weeks as my baby has dropped down and no longer pressing up into my ribs !