Help!!!! is my pony in foal? anybodys opions would be great!?

right. I got my pony trained and broken in last june in a riding school. She was there for four weeks and was with other horses. At the end me june i brought her home. I cant remember the last time she was in heat and over the part two months her belly has gotten bigger and deeper. She is in a field smaller than half an acre, which she shares with another mare. I ride her nearly every other day. So i what is every body's opions on this? Is she in foal or did she just get really fat really fast? Thanks for all answers