I could have written your post on any of a number of days over the past year and a half. DS is extraordinarily social (to the point that even as an infant, he'd be miserable and crying and cranky at home with me and all smiles and happy as soon as he got into the grocery cart). We have a baby/toddler lab at the U here too, and the first time we went, he did nothing but cry. The second time, he pooped as soon as they took us in, and we nearly missed our entire appointment (only 20 minutes long) b/c I had to take him all the way back to the car to change him since I didn't think I'd need the diaper bag for a 20 minute appointment. The third time, he agreed to sit, but not for long, and basically refused to cooperate. I can't believe they ever let us come back after those first three times.
The only "social" outlet he has other than an occasional playdate is library story time. At the park, at the mall, at playdates, at the gym he runs off and has fun. At library story time, the one time I can sit in a chair away from him and watch him enjoy himself from a distance, or (heaven forbid) even read a book and zone out, he won't leave my lap. I have to sit on the floor with him and practically force him to do the motions to the songs. Not fun on day 4 of Heavy Flo.
IDK what it is, but they seem to have a good radar for just what's going to put mommy over the edge. Today it was poop in the naptime diaper after being potty learned (and no poopy diapers even overnight) for six months.
You're not alone. I know it'll get better (for us, it already has in some ways, other than today's random poop, which really isn't a tragedy, and could've been worse by being done in pants). It's definitely hard to remember that some days, though. Hang in there! Tomorrow will be better!