I had a similar experience when I joined grad school in 2007. I couldn't get through, or even really understand papers, even though I thought I was a good reader. I also had to "learn to read" again. For me, it was about 1 semester of focused effort to rebuild the skill. I was in my 20's then.

That said, if you don't see (or haven't seen) noticeable improvement over 3-5 months, that is probably a signal to pay attention to.