I can't tell if my chihuahua is pregnant?

One morning, as every other morning, I put my female chihuahua on her leash outside for 10 to 15 minutes to potty. When I went to bring her back in the house, I noticed that someone in the neighborhood let their dog run loose (which is against park regulations) and was all over my female. I didn’t see them doing the deed but she was whining and her hair was wet from the back of her neck all the way down her back to her tail. Just so happenes, my female was in heat but she had a very minor cycle and bled very little. The day before the “attack”, she had stopped bleeding but the day after she bled a little then no more. This was approximately 3 weeks ago. Over the past two to three days, she has been very lethargic, not eating and no where near as hyper as she usually is. I notice that her nipples are a little pronounced but, never having a chihuahua before, I’m not sure if that is normal for her age (3 years old). I don’t see or feel any changes in her tummy but I’m wondering if it would be too early to tell anyway. Any advice? I’ve never had one on one experience with a pregnant dog and I’m not sure what signs to be looking for or if maybe she is just sick. Please respond to my emai cassie20mack4@yahoo.com. Thanks!