The strong, broad head makes me immediately think of the Staffordshire Terrier..
Whatever she is, she looks sweet.
You might want to look into those DNA tests that they offer now, that can figure out what the breed mix is. I heard it costs some money, but worth it. I met a man on another forum who had a shar-pei mix and was asking what we thought the other breed is.. Everyone was dead set on a lab or a great dane...and it came back that he was mixed with weimaraner.. We all were shocked and it was fun to learn.