Like most colubrids, Thamnophis have a Duvernoy's gland that produces a toxic secretion (no, it's not just toxic saliva). Documented symptomatic bites in humans have resulted in swelling and pain. This was usually as a result of prolonged feeding bites. Headaches have been documented after other colubrid bites (eg Boiga dendrophila). In the case of garter snakes, headaches and nausea in the absence of swelling and pain would suggest that it might not be a result of envenomation - could just about anything from dehydration to some flu-like bug. Of course, there is no way of being sure.