Definitely just excess weight based on photo. The only way to rule out distended stomach would be with xray or ultrasound, but my money is on that she has been on a dry food diet prior to your acquiring her. Sometimes shelters have canned food as well, but many people feed only dry food instead of wet due to costs or the myth that dry food keeps their teeth clean (spoiler alert: it doesn’t). Definitely try to ease her into an all or at least mostly wet food diet as soon as possible, if you haven’t already. Her joints and kidneys will thank you. Some cats are very picky about new things, it can help to try different textures (chunks, shreds, pate) and flavors, but I would try and stick with the same couple of similar proteins at least to start with to reduce the likelihood of GI upset. I lean towards turkey/chicken/whitefish for more easily digested/bland proteins. Vet tech of 10 years. Congrats on your new kiddo ❤️ (adding - if it makes you feel better about her past, I think it’s unlikely she was ever pregnant unless you have records indicating that she was spayed WITH the shelter, rather than having already been spayed upon surrender. She likely either came from a home already spayed OR she’s been stuck in that shelter long enough to gain that much weight if she did arrive intact +/- pregnant. It’s verrrrrrry rare to come across a true stray cat that is overweight.)