FAQ

Yes. The dogs who were chasing Lady down the alley were chasing her because she was "in heat" (which is when a dog or mammal is ovulating). Then at the pound, Lady felt betrayed about the rest of his girlfriends, because she had gone 'all the way' with him, which was unhonorable for her. The real reason why Jock and Trusty proposed to Lady was because they knew she was pregnant, and cared about her, and they wanted to protect her Honor in the dog community. Also, Lady gets pregnant with the puppies. At the end, it is shown that it is the Baby's first Christmas, so therefore Lady would logically have gotten pregnant that night with Tramp.

Walt Disney subtly put in those hints, so that the younger audiences wouldn't be able to figure it out, but older ones would. This was actually quite daring for its time considering the Production Code prohibited pre-marital sex in Hollywood films without negative repercussions (not that you can judge animals by human standards).