If memory serves, his last big trail off was around the departure of Mike Flynn as National Security Advisor.
So why now?
It's hard to say. We should not rule out the possibility that this is simply noise or a drop-off that will bounce back to a significant degree. Whether it's temporary or enduring, for the cause I would put my money on two related developments.
First is Obamacare repeal, which is now tied to tens of millions of people losing their health insurance coverage. "Access" versus "coverage" may give true believers something to say. But no one but true believers buys that. There are legitimate questions about the roll of government and cost tied to Obamacare. But no one buys the idea that tens of millions will lose coverage because the government was somehow forcing it on them in the first place.
The other cause may be less obvious. I believe President Trump's inability to move the bill forward may be hurting him as much or more than the substance and consequences of the bill itself. That may sound counter-intuitive. But public opinion doesn't always work in straightforward ways. First of all, the two drivers can be affecting different groups of people (that's certainly part of it). But these two drivers can, paradoxically, affect the same people. Support for a President can be driven in large part by his or her effectiveness, their ability to drive an agenda forward, even when people do not necessarily agree with the agenda itself. Paradoxical but true.