For me, it was one of my first "signs" something might be up: with DS, I think the increased blood flow etc, actually made sex worse, it was very uncomfortable at the start, but was fine after a bit. With this baby, sex has been great, the extra blood flow makes things way more pleasurable.
And the change in both cases was almost immediately after conception. Though TBH, I have been too tired to really even think about DTD for the past couple weeks. Looking forward to getting some more energy,maybe in the 2nd Trimester.