What Causes Red Sores Around The Penis Head?

Presently I have small red sores around the head of my penis (I don't have a foreskin). These sores can also appear on the top half section of the shalf of my penis & can scab at times. I've had the same partner since the symptoms first occurred & I assumed I caught this STD from her. The sores don't break the skin until I have sex or masturbate. If we don't have sex the sores seem to dry up & clear. After sex they can bleed a little & more tend to appear again days later. Then it can be some weeks before they settle & almost clear totally although there is still some redness. There doesn't appear to be any bumps or lumps so I get the feeling it may not be herpes. I have been for a test before with my previous partner & came up clean. Now I am being accused of passing whatever this is on to her. Just can't work out why I would come up clean after seeing a doctor then have this happen some months later. It appears I may have had something wrong with me before the first out break occurred. Now I know I should see a doctor but would really like some idea what might be going on before I go. I really need some advice please.