When DS now 8 was in K, he tested at 4.2 at the beginning of the year. The next quarter he was higher still at 4.9. The rest of the year he stayed level. I too questioned this, but as he was allowed to supply his own reading material that part wasn't an issue.
A very wise teacher put forth the idea that he was busy learning other things, filling in the gaps, making monstrous leaps in math and probably wasn't focused too much on free reading. She suggested that I let it lie for a bit and see what happened.
It turned out to be excellent advice. The following year, he tested in at 7.3. Over the last two years, we've had a few more of these lulls in various subjects but he always rebounds in a big way.
I would consider letting it lie for a bit. Even if it does turn out to be something, she is far enough ahead that a little lull isn't going to affect the long term.
