HELP ME!!!!!!!!!!!!!!?

I went to go check on my runt chihuahua (She is 6 years old) at around 10:30 this morning. I usually take her and her wiener dog pal (Who she hates) out to "go potty" and she didn't run out like she usually does. She was just laying on her bed. I picked her up and put her outside so she could run around. She wouldn't put pressure on her back right leg, she was just holding it in the air. I brought her up to my mom (Don't worry, I'm 13) so she could check her out and she decided she needed a bath. We took her to the sink and we found a black raised spot on that foot. I have to admit that I spent the last 2 days at the miss georgia pageant rooting on my sister. I'm not sure if they had food or water for those days because I came home and they didn't have any. My Uncle took care of them and he's not home so I can't ask if he did or not. My dogs live in their own really big room that leads to our laundry room. Our chihuahua likes to escape from my wiener dog (She's not mean, she's just annoying because she's still a puppy) by going in to the laundry room through a crack in the door just big enough for her to fit through and she's cut herself before trying to get in (That time she got really sick but she's perfect now) Everyone goes through that room at least once a day so it's basically impossible to keep the crack shut. I love my chihuahua so much and I need to know what's wrong with her soon!!!!!!!!!!!! PLEASE HELP!!!!!!!!!!!!!!