In that case I say she's gravid. I've not kept garters so I can't speak for how much they eat, but keeping a male & female together is asking them to breed & they're
mature enough to do so, though it's generally better to keep snakes apart until they're fully grown...early reproduction can stunt the females growth or even shorten
her life span. Hope all goes well, & hope you can get tiny enough food for the babies you're likely to have.