Well I can see why you’re wondering, that does seem like a big change in only a couple of months. It could be that she’s just gained weight but definitely need to know either way because if she is pregnant you don’t want to cut back on her feed thinking she’s fat.
I find the previous seller’s statement the she “didn’t think so” to be a little worrisome. If someone asked me if one of my horses might be pregnant I could say for certain that the answer is no. No stallion at my barn, horses have not been to a barn with a stallion, no loose stallions in my area. I would never say “I don’t think so.”
Fingers crossed she is not. Keep us posted.