I had a platysma done 6 months ago. I noticed about 3 days ago it's red behind my ear, could this be an infection? (Photo)

Most likely, the issue in the photo represents a stitch abscess and you should bring this to the attention of your Surgeon.  This is not likely to be a major issue, but requires more than Neosporin.  

Thank you for your question. I recommend following up with your doctor for a formal evaluation and after care recommendations. It looks as if the sutures or stitches have created some fluid buildup. 
 Wounds need a combination of:
     1) healing lasers and light such as aerolase or laser genesis and LED therapy with light stim
     2) topical growth factors, stem cells, antioxidants
     3) oxygen facials
     4) platelet rich plasma and A cell topically
     5) sun protection
I recommend getting a formal consultation with an expert.
Best, Dr. Emer

Its likely a deep suture that is making its way out - often as the body tries to dissolve these deep sutures, it may create a little pocket of fluid and "spit" it out.  Its not emergent, but you should let your operating surgeon know such that they can make sure there is nothing else going on.  Best of luck, and congratulations on your procedure!