Good for you for seeing the doc to follow up. There COULD actually be something wrong, and it's sooooo refreshing when a pt is proactive. :) :) :)
That said...if nothing's wrong...here are a few other things to think about:
1) Some of us have more control over our autonomic responses then others. Ghandi could literally heat his feet up to over a hundred degrees when walking in the snow. I can slow my heart rate down to below forty if I concentrate on it. SOME women, when they strongly desire a child actually start lactating. (There is debate over whether or not this makes them more fertile or less...i THINK because it's roughly 50/50 on whether it's just their mammary glands, or their whole body...aka the body thinks it's pregnant so it can't actually get pregnant/natural birth control).
2) Most of us have had sex so often that we don't get that full/heavy/painful feeling in our breasts anymore when we're having (ahem, more to the point, AFTER) sex. However, if you're having sex a lot more frequently the simple act of friction + increased blood flow, + more often then usual can also cause those symptoms.
3) So many false negatives are rare, but I'll cross my fingers for you!! Good Luck!