My dog has a really bad rash?

I have a chihauhau. A few months ago she was itching and rashes and lost her hair. I took her to the vet and they did a skin scraping. She had decodermic mange. I payed for her first dipping and her antibiotics. I had to take her every week for a dipping. My cousin who used to be a vet said they were charging way too much for the dippings. $50 every week. So she bought this really expensive at home dipping and gave them to every every two weeks like it said to do. She finally grew her hair back. The vet told me she had a suppressing immune system and it could be because she is about to go in heat. Well now she has a big rash on her chest and little yellow bumps down there on her. She is itching a lot but not losing hair. I talked to the woman I got her from she said she kept a puppy too and that her dog is in heat so does that mean my dog Martini is about to go in heat? And that is why everything is started to break out? I can't take her to the vet until May 1st. Should I do another dipping? I still have about 6 of her antibiotics that the vet gave me and I missplaced them but I found then a few days ago. Can I give them to her? And I give her childrens benadryl like the vet said for her itching. I don't know what I can do until May 1st. I hate seeing her itch. It makes me feel like I've done something wrong and I hate seeing her go through all this.