she must have been working the corner all day while you were at work lol
Congrats thats exciting though!
Its completely possible for a female to retain sperm for awhile. You said you "rescued" her so maybe before that she was paired with a male and didnt feel secure enough to lay at the time. Once shes been with you for awhile in a better environment it was safe enough for her body to release to sperm. Thats just my thoughts on it.