How can i tell if my Dog is pregnet?

I have a year old English Mastiff and she just came out of the "heat" stage, while we where out, we have chained her out side, when we retunered we swaw a very lard black pit bull, and that has be3en about 2 weeks ago maybe 3, now her nipples are starting to largen out, ( she isnt nutered or spared becaus she is still young and we wanted to breed her once or twice) and she hasnt wanted to play *** much all she doess is eat and lay on our bed, does this mean she is pregnat? or justt normal.