Finney, Could be, but difficult to tell from here. If it's significant weight gain and truly muscle mass, then you'll notice the difference in toning, particularly of your upper body.
Sometimes though, when running approximately at the 30 mile/week rate, runners can simply overeat.
On the other hand, not knowing your history etc. it may be that simply you're trying to get to / stick to, too low a weight. Just as an example, in the last year, I have got my weight as low as 12-5 for some events, but generally, when I'm fit and things are going reasonably well, I'm around 12-8 / 12-9 and frequently enter races at this weight.