I hope I can help ease your mind here. Please though understand that online, no one can state anything with certainty; all I can do is offer my best guess based on what you've written...and uploaded.
It is true that a picture says a thousand words, and I see the slight redness to which you refer. Its very hard to say what it could be though online. Its possible that the oral sex irritated your penis; that is easy to do if the womans technique is not the best and her teeth were...rough. Factors against that are the time frame; I think that would have shown sooner had that been the case.
Its possible you may have a very slight fungal or even bacterial infection of the head of the penis called balanitis. Usually we see that in men who are not circumcised, but it can happen in those who are. Balantitis, IF fungal, can be treated by over the counter anti fungal creams. HOWEVER, I do not know 100% that it IS a fungal infection (or even that it is Balantitis). Fungal infections tend to more red, and more spread out.
Balantitis can also be caused by bacteria; if this is the case it would need antibiotic ointments or oral medications (depending on severity).
Could this be a very very mild herpes? Herpes simplex is a very common virus' most of America (well, approximately 80%) has been exposed to HSV type 1; and 40% of people have been exposed to genital herpes. Herpes lesions are usually blisters, and often quite painful...but some people have it mildly, so mild they are not even aware of outbreaks. The time frame is there, the incubation period right, so I cant rule that out completely, but I think it very very unlikely from what I see.
Here is a helpful website regarding balantitis: http://www.nlm.nih.gov/medlineplus/ency/article/000862.htm
Images of BACTERIAL caused Balantitis: http://www.dermnet.com/Balanitis-Bacterial/
You will see it refers mainly to uncircumcised men; these pictures are of SEVERE cases. Yours if this even is balanitis is VERY mild.
I am pleased you got STD testing, and you are getting those results tomorrow. If this discomfort of yours continues, I would suggest getting it re-evaluated. Herpes lesions will fade on their own in a week or so, if this is mild herpes (very unlikely)
This is a chart of STDs and their signs and symptoms in both men and women: http://www.fwhc.org/health/std.htm
I hope I was able to make you a little more comfortable,