She could have bagged up because she now has spring grass? But as a 2 year old, she has presumably not bred before so unlikely to be a carry over from last year's lambing. It's highly likely she is pregnant if her nipples and udder are noticeably bigger than the rest. I wouldn't worry. Just keep an eye on her. If you have sufficient grass she won't need any additional feed as spring grass is highly nutritious. If you can see her udder now, you will definitely notice when it fills up even more as lambing is imminent.