All the symptoms of a cold sore (inflammation with pus coming out, then yellow scabbing and healing with 1-2 weeks), however the test for herpes simplex was negative. what can it be else?

There is a false neg test in 10 out of 100 people, it takes about 3 months after first infection for the test to be positive or improper collection of specimen are some of the causes. Or it might be another kind of infection eg bacterial. Hope you do well.

The blood test for HSV1 remains negative in about 30% of people with HSV1; probably that's the explanation. If it was a swab test (i.e. PR test for HSV), then perhaps it was done too late; that test typically is positive for only the first 2-3 days after onset of a herpes outbreak. Consider retesting within a couple days of onset. Good luck.