Stipules or calyx? Preflower?

I had a similar experience in my last grow - single female calyx sacs like that swelled and were all female (i.e. pistils pop out). The picture looks the same though it's hard to see close-up. Watch it over the next few days/week but I don't think you've got anything to worry about there. Preflowering is simply the plant showing its sex at a few nodes during veg growth - before it has really started flowering (i.e. throughout the plant).