Why does my mare still look & act pregnant?

I've had a 4 yr old mare for several months now... she was already pregnant when she came to me (rescue). She's a sweet girl who foaled a filly on May 6th. The foal and mare did great with no complications. She's current with vaccinations and worming but for some reason she still looks pregnant. She is a little on the lazy side, which I'm not sure if this the norm for her as she was pretty far along when she came here. Her filly was large for her, and continues to grow at an astounding rate, she's very active to say the least but Mom is still carrying a lot of weight. If I didn't know better I'd say there was still a baby inside her lol. Any insight is greatly appreciated.