I agree with KATE535. I'm on pregnancy #4 and I have all of the same things. The orgasm causes contractions.....no biggy. While I'm pregnant I am very sensitive to even bathing. It causes yeast infection almost immediately!! Therefore, DH has to do the pull out method because his sperm causes the itchy and uncomfortable feelings. I think you're ok but definitely discuss with your doc at your next appt.
Don't worry, I have been going through the same thing. But this is my third pregnancy and so I'm not worried about it. Here's why...
I asked my midwife why, after sex, I am tender and sore, and she said it's just because you are really sensitive in that area when you are pregnant. She was like ''oh, that's normal !''. I too feel burny after weeing after sex, but it's becuase the area is so sensitive, it becomes a bit inflamed after sex, and for a while after too, hence the tenderness and burny feeling (also due to the acid in your wee).
As for your belly/ bump dropping low, that happens to me straight after orgasm too. It goes so hard I can't actually move comfortably at all until it passes. But when you think about it, an orgasm involves the contraction of the vagina, cervix etc, so when you orgasm when your pregnant, this contracting of your sexual organs resembles a labour-contaction. However, this is not a sign you are going into labour. This is nothing to be worried about since it is just your body reacting to orgasm/ sex. I too was a little concerned and so looked it up and found orgasm is actually good for your baby as it increses the flow of oxygen into the placenta. So it's very healthy.
Hope that helps !!