This sounds like she may have a hotspot from the collar. What can happen is that the skin gets moist under the collar and if she scratches or gets a bug bite or something that irritates that area it can create a way for bacteria to start growing and create an infection. It would be best to remove the collar for now until it heals. If you can you need to shave the hair from the area, this will allow for better air circulation so it can dry out. Once it is shaved then you can use a mild anti-bacterial soap to clean it with, make sure that you rinse it well. Try to get all of the slimey feeling stuff off of the area. Once it is cleaned well you can use some neosporin on it to help it heal. It will look worse before getting better so don't be too alarmed. She may need oral antibiotics to get it completely healed. You can also use cold compresses on the area to help cool it down. If the area feels like there is a pocket or balloon under the skin she may have an abcess,which could come from a bite of some type or possibly the collar if it. If this is what is going on then you need to get her seen by a vet and get the abcess opened up and drained and get her on antibiotics right away. Her heat cycle should not have caused this to happen.