Little, Sensitive Pimple Kind Of Thing On Penile Head Which Is Visible During Erection. What Could This Be?

Hi,i have got this little pimple kind of thing on my penile head which is quite visible when my penis is erect but not so much visible when my penis is flaccid,after i ejaculate it recently seems to be alot more sensitive than usual,the increased sensitivity is mainly coming from this pimple kind of thing,the sensitivity is worst on the pimple and not as bad around other areas of my penile head,i did go to a clinic to get checked out because i dont seem to be enjoying sex as much as i once did because of this sensitivity,i was checked for chlamydia and it came back negative,the doctor physically assessed my penis but i was quite nervous and it slightly shrivelled up and the pimple werent very visible at all and he then went on to say my penis looks perfectly fine,i just dont know what to do i have been applying thrush cream to my penile head recently because i thought it might be that but its not exactly helping,if you could try and diagnose my problem it would be much appreciated.