Another knee diagnosis needed

I did one of my longest training bricks on Sunday(17 hilly mile bike and 4.5 mile walk/run) and starting yesterday I noticed some discomfort on the back left end of my left knee. I wouldn't go so far as to say that it hurts but it bothers me a bit when I completely straighten my leg.

I've been lucky enough to get through the spring and summer w/o injury but now this has me concerned b/c I have my longest tri coming up on 9/8.

Has anyone had a similar issue. Am I jumping the gun as the discomfort starting only yesterday. Should I see the doc if it continues. Will insurance cover a PT?

I felt OK during the brick and even that evening and the day after...

Any advice?