Previous installments of Feel the Burn can be found here. “Working Out in Your First Trimester” is here and "Working Out in Your Second Trimester" is here. I quit the gym today. I had planned to go until next Tuesday, when I'd be 36 weeks pregnant and ready to slide blissfully into holiday-related indolence and two different kinds of stuffing, but I knew yesterday that I was done. Or rather, I…