Help Neutered Chihuahua trying to mate with my new female jack russel mix!?

Okay lol.. I recently rescued a female jack russel/beagle mix from a high kill shelter- she was due to be euthanized the next day so naturally I adopted her immediatly- anyways apparently she is not spayed as she went into heat last week... I made an appt but cannot get in until April- which is fine as she IS an indoor dog, however my NEUTERED male chihauhua (who's been fixed now for well over a year) thinks differently... My husband just came home and the dogs were gated in the kitchen for the night so I had no clue what was going on- but he caught them tied together- other words having ... WTH?? LoL! Now Lulu my female freaked when he came in and broke the knot- blood going everywhere :-/ We've cleaned up both dogs and aside from obvious swelling I can't see any injury to either of them... I want to call the 24hr vet ER but I'm almost embarred they'll think I am pranking them... I am concerned- I KNOW my boy is fixed, my vet would never take my money and not do it lol that's completely out of the question... But... Is this common behavior?? I have never seen such a thing and I have had animals my entire life- I also volunteer at a cat shelter w/ 100 cats and none of them do this to each other... Also-forgive my foolishness but I know people who have had vasectomies fail, please tell me she cannot get pregnant?? And lastly both of them seem fine but I'm concerned about injury- should they be seen tonight?? My girl has an appt to be spayed- it's just this is all happening so fast, and everyone is backed up w/ spays- the earliest I could get was June... Just trying to figure out what's going on in my house :-( I have them seperated now until I know more about this- I am on the waiting list to get her in in lieu of any cancellations- this is crazy.... Oh my!