Your doctor is right. The red color, if it is blood, could be coming from your nose or any of the areas adjacent to it. Your doctor's recommendation is appropriate. If it doesn't cease in another couple weeks, you should have a good exam by an ENT specialist.
You refer to hacking up phlegm from your lungs. That is definitely abnormal for a healthy 28 year old. If you smoke, you should give it up immediately.
I am 28, too, having the same problem with some blood occasionally in mucous from the back of the throat. With nasal dripback and a clear mucous coughed up from my lungs directly.
As I was clueless, I have recently taken a gastroscopic exam and it showed some oseophagitis caused by an acid reflux from the stomach. I was told, that this might be one source (among the others like sinusitis) of the slight amount of fresh blood in the throat. You may ask your doctor about it. Hope it helps somehow.