strangest thing

Question
I know your not a vet but I would like to ask a question.  I took my cat to the vet the other day for an absess.  We took care of that (under right side of jaw) drained it, 10 days of antibiotics, cat better and sore looks great but.......they gave him a shot fo antibiotic before senthome in his neck.  I got home, all fine, the next morning i noticed a completely bare spot on the back of his neck.  No sign of injury just a few scrapes best I can figure.  I had no clue what it was.  The next day I looked at it and now has a spot totally red that looks like blood but its not just blood red skin.  Now, it is not totally covering the bald spot.  When I first saw it it was aperfect square shaped.  So darn strange, no cat fight as he has been in house since i took him to vet.  Don't understand why it looks worse the next day but only part of it.  Never seen any skin condition like this and for it to come on in one day in the same spot the shot was given concerns me.  Any thoughts before I go to vet for the tune of $100?

Answer
Hi Jill!
I could be wrong, but it sounds like your cat had a reaction to the antibiotic shot.  If it were me,I would give him some Benadryl. I am going to give you instructions below, be sure to read carefully.(such as be sure to give in pill form)  If this doesn't help, and it gets worse,then you may need to go back to the Vet but my guess is that you won't have to.
God Bless,
Becky

Benadryl for Dogs or Cats
Benadryl is a brand name for an anti-histamine (diphenhydramine).  Although it is considered fairly safe for use in dogs and cats,  please be aware that it does have potential adverse side effects.  It may interact with other meds that your pet is taking, so do not use it without first consulting your veterinarian. You should also use extreme caution using Benadryl in dogs that have glaucoma, prostatic disease, cardiovascular disease or hyperthyroidism.  

Benadryl is quite effective when used in proper dosages.  
Stops itching
Controls vomiting
Good for motion sickness
Can be used as a mild tranquilizer
Good for snake bites
Useful for vaccination reactions
Good for bee stings and insect bites

Be sure to use only the plain Benadryl formula. Giving benadryl with other combination of meds could be fatal, so be sure to read the box.

Liquid Benadryl is too high in alcohol content to be safely used for animals.  Please use capsule or pill form.  

Dosage every 8 hours
Dogs under 30 lbs and Cats :  10 mg
Dogs 30-50 lbs:  25 mg
Dogs over 50 lbs: 50 mg