first time sex, burning sensation

Hi ,

I just had first time sex 2 days ago. My partner said she is clean and I fully trust her (it was not her first time) . I also used a condom (latex).
I am the most paranoid person on earth :-P , and I'm so worried that I might have got some kind of STD, crabs. I got some itchiness on my legs (where I was touching her legs when I was on top ) but I'm guessing this is because we did it for like 45 min and she had shaved 1 day ago so hair was growing a little bit ( I was feeling it ). just today, I felt a slight burning sensation when masturbating ( just at the tip of my penis ). I always tend to over react so I'm guessing maybe it's all in my head.
So my question is, is it normal to feel different after 1 st time sex. like my penis being more sensitive ? also, I always think about this night and I'm hard like 70% of the time during the day !!!!!
I'm totally confused, pleaaaaaaaaaaase help
I met a doctor today and she told me not to worry, but I forgot to tell her about this burning sensation (which is not bad at all.. it lasts like 30 seconds or so )

sorry for the long post.. I tried to be as detailed as possible

MANY THANKS