How old is she? Has she been spayed? Going to the vet can be stressful for buns, but the alternative in this case is that she has developed an infection that could kill her if not treated.
Is your other bun neutered? If one or both are not neutered, they could be biting eachother out of sexual frustration. That may be how she got the wound. I am VERY sorry you lost your other bun during neutering, but please understand that that is very rare and it is very necessary for a house bunny – ESPECIALLY a female bun, who has an 85% chance of dying from uturine cancer if not spayed – to be neutered. Take bun to the vet in a cat carrier with a nice soft towel in it. She will be fine. The vet will probably send you home with an antibiotic that you will have to give her. Do you have another person in the house who can help you give the medicine? You will most likely have to place a dropper of liquid medicine into the corner of her mouth and squeeze liquid in for her to swallow. Some buns like medicine and take it easily, some hate it. Have the vet show you the easiest way to do it. And while you’re there, talk to him about getting her spayed. Good luck and try to relax – your bun can tell if you’re stressed, so be a soothing, calming bunny momma and things will go much easier.