What Causes Hardness Of Penis In Flaccid State After Having Oral Sex?

Hi,
Im an 18 year old guy. A about 1.5 months ago i received really rough manual (hand) stimulation from my girlfriend. When i got home my penis was swollen and hard in the flaccid state. In about a week that went away and it was normal. 10 days ago she gave me oral sex. for the past 10 days my penis has been hard in the flaccid state and doesnt keep an erection quite as hard as i'd like, it still gets mostly up though. I've never had any problems like this before. 4 days ago i started taking L-arginine and ginseng to see if those would help my blood flow and help it heal.
What could be the problem?
How could i fix it?
Are the two supplements i'm taking going to help or hurt it?

thanks