declining canine health question

Question
Hello, my question relates to my 6.5-year-old male St. Bernard (Sammy).  I returned from a weekend trip Sunday night to find a 2-inch area of my dog's right cheek matted, wet, and oozing with pus. He will not let me examine it closely, but it smells horrific and he wants to scratch it.  I also noticed small scabby areas below and medial to his left eye, which come off with the fur when the fur in the area is stroked.  The skin underneath is red and swollen.  Thirdly, my dog has not been able to keep down any food (or anitbiotics I put in his food to try to attack the infection over the holiday yesterday).  His vomit is of a thick watery consistency, with the exception of last night, when he vomited his dinner 10 minutes after ingesting it.  My other 2 dogs seem all right, and my pet-sitter hadn't noticed anything else out of the ordinary when she was here.  I do give him 2 aspirin per day for his arthritis, but I didn't give any with the antibiotics.  The weather has been hot and humid lately, but he acts this way even indoors where it is cooler.  Do you think it is heat/stress-related or otherwise?  I am trying to determine if I should see my vet because he can't keep down food and I am afraid of giving antibiotics on an empty stomach.  Then again, he just had a full chemistry and urinalysis profile in March, with no problems reported.  My main question is:  Is there a condition or disease that could cause a serious, measurable change within that amount of time?  I would greatly appreciate any information you could share about what could possibly be bothering my big puppy. He has such a sweet demeanor and I am afraid he is hiding any pain he may be in.  I just don't want to be ignorant if he is suffering.  Thank you in advance for helping my dog.
Denise  

Answer
YES you should take him to the vet.  The area must be clipped and cleaned or it will not heal properly or just become reinfected.  Just putting him on antibiotics of your choice is not appropriate as you should have the vet examine him and prescribe antibiotics based on his or her findings.  The vomiting on the antibiotics is not a good sign and means he can't tolerate them.  Or he could have even developed a stomach ulcer from the aspirin you are giving him combined with the stress of everything.  Regardless of normal bloodwork a few months ago, he is sick today and needs medical attention.